🕷️ Crawler Inspector

URL Lookup

Direct Parameter Lookup

Raw Queries and Responses

1. Shard Calculation

Query:

Response:

Calculated Shard: 51 (from laksa153)

2. Crawled Status Check

Query:

curl -X POST \
  'http://laksa051.int.ahrefs:8124/' \
  -H 'Content-Type: text/plain' \
  -H 'X-ClickHouse-Database: crawler3' \
  -H 'Authorization: Basic YXBpOg==' \
  -d 'SELECT getAhrefsURLFromUnparsed(src_unparsed) AS found_url, ifNull(toUnixTimestamp(download_stamp), 0) AS crawl_time, ifNull(toUnixTimestamp(props_url_first_seen), 0) AS first_indexed_time, download_http_code AS http_code, src_unparsed AS src_unparsed, src_root_hash AS src_root_hash, history_drop_reason AS history_drop_reason, meta_title AS meta_title, meta_descriptions AS meta_descriptions, attrs_boilerpipe_text AS attrs_boilerpipe_text, attrs_markdown AS attrs_markdown, attrs_readable_markdown AS attrs_readable_markdown, meta_canonical AS meta_canonical FROM crawler3.page_info_local FINAL PREWHERE (src_root_hash, src_unparsed) IN ((getAhrefsRootHashFromUnparsed(getAhrefsUnparsedNoserviceFromURL(\'https://www.cnn.com/2022/01/19/health/pcr-vs-antigen-tests-covid-19-wellness\')), getAhrefsUnparsedNoserviceFromURL(\'https://www.cnn.com/2022/01/19/health/pcr-vs-antigen-tests-covid-19-wellness\'))) FORMAT JSONEachRow'

Response:

{"found_url":"https:\/\/www.cnn.com\/2022\/01\/19\/health\/pcr-vs-antigen-tests-covid-19-wellness","crawl_time":1776032595,"first_indexed_time":1746122997,"http_code":200,"src_unparsed":"com,cnn!www,\/2022\/01\/19\/health\/pcr-vs-antigen-tests-covid-19-wellness s443","src_root_hash":"2312100192101524051","history_drop_reason":null,"meta_title":"PCR vs. antigen tests: Which you should take after having Covid-19 | CNN","meta_descriptions":["A PCR test is considered the gold standard for detecting coronavirus infection after exposure to Covid-19 or while you’re experiencing symptoms. Coronavirus test experts explain why the rapid antigen test is best for knowing if you’re still infectious."],"attrs_boilerpipe_text":"CNN\n — \nA PCR test is often considered the gold standard for detecting coronavirus infection. It’s the test some employers want employees to take before returning to work after having Covid-19, and the one some people scramble to schedule to know whether they can travel or stop isolating. But experts say the PCR test isn’t the best one for every situation.\nPCR – or reverse transcription-polymerase chain reaction – tests\ncan detect small amounts\nof the coronavirus’ genetic material in a specimen collected from a human. The test then works by amplifying, or making copies of, that genetic material if any is present in a person’s sample, according to the US Centers for Disease Control and Prevention.\nThe test’s amplifying property enables it to detect very small amounts of coronavirus in a specimen, “making these tests highly sensitive for diagnosing COVID-19,” according to\nthe CDC\n. While this sensitivity can be advantageous for detecting coronavirus after a recent exposure, it also means PCR results can be positive even after you’re no longer contagious.\nAd Feedback\nAd Feedback\nA PCR test might say you’re positive for coronavirus for three or four weeks after you’ve recovered because it’s still “picking up past infection and the small fragments (of the virus) are still being amplified,” said CNN Medical Analyst Dr. Leana Wen, an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health.  After seven to 10 days, “that PCR test is not an appropriate test.”\nRegardless of your situation, here’s what you need to know about the differences between PCR and antigen tests (called\nrapid lateral flow tests\nin the UK) and when you should use them.\nThe prime time to take a PCR test is when you have had a known or suspected exposure to someone with Covid-19 or are experiencing symptoms, and you want to find out if you have a coronavirus infection, said Dr. Albert Ko, the Raj and Indra Nooyi Professor of Public Health at the Yale School of Public Health.\nKnowing your Covid-19 status as early in an infection as possible can help you figure out whether you’re infectious to other people, what to tell recent close contacts for their own safety, and what to share with your doctor so they can talk with you about your symptoms and prescribe any medical care if needed, said Emily Somers, an epidemiologist who holds professorships in internal medicine, environmental health sciences, and obstetrics and gynecology at the University of Michigan’s schools of medicine and public health.\nThat’s important because “up to two days before symptoms, or the one day or two days after symptoms (begin) people can have the highest risk of infecting others,” Ko said. When people are first exposed, it takes time for the virus to replicate enough for a test to pick up on it, Ko explained. PCR tests detect it earlier in the course of infection after exposure than the less sensitive rapid antigen test, which works by detecting a specific protein on one of the coronavirus’ spikes.\nThe US Food and Drug Administration has authorized use of both PCR tests and\nrapid antigen tests\nin laboratory-based or point-of-care settings (such as a physician’s office, urgent care facility, pharmacy, school health clinic and temporary locations like drive-through testing sites). Antigen tests are also commonly available as self-tests.\nContrary to what some still think is best, a rapid antigen test (or rapid lateral flow test) is what you should be taking after you have had Covid-19 for several days and want to confirm you’re probably no longer infectious to other people. “‘Infected’ means that I have the virus in me,” Ko explained. “’Infectious’ means that I’m shedding enough virus to infect somebody else, and that only occurs during a peak. It follows a curve after someone has been exposed.”\nThe CDC’s\ncurrent isolation guidance\nis that if people have access to a coronavirus test and want to take it, the best approach is using a rapid test toward the end of their five-day isolation period. Those who continue to test positive at that point should continue isolating until they reach 10 days after their symptoms began. People who test negative can stop isolating but should wear a mask around others until day 10.\n“Even though the rapid (antigen) test has low sensitivity and is inferior to the PCR test to tell you if you’ve been infected, because of its lower sensitivity, it picks up only viruses at a higher level and probably levels that are more infectious,” Ko explained.\n“At this point, we don’t have an FDA-cleared test that can tell us exactly” how much virus is needed to be infectious to other people, Ko added. But evolving evidence has led public health experts to think that rapid tests do better than PCR tests at capturing when people are exiting the period of maximum infectiousness, since PCR tests are too sensitive, he said.\n“We have other data to indicate at what point somebody is no longer infectious,” Wen said. A\nUK preprint study\npublished in December 2021 found that after five days of coronavirus infection, 31% of people were still infectious. On day eight, that percentage dropped to 11% and continued to decrease over the next several days.\n“Having the presence of two rapid tests – two days in a row – that are both negative is a very good indicator a person is no longer infectious,” Somers said.\nThis doesn’t mean that being contagious at this point is totally impossible – it just means that once you have had Covid-19 for several days and then test negative with a rapid antigen test, the viral RNA is likely at levels low enough to not be infectious (even if you still test positive on a PCR test).\nThere have been many online reports of employers requiring people to show proof of a negative PCR test before they return to work, which is “probably not coherent thinking” and has people not returning to work until weeks after their initial rapid antigen or PCR test, Ko said.\n“I would highly urge individuals to contact their HR (human resources department) – or if there’s a medical or Covid lead on their teams – and escalate the issue because it’s not reasonable,” Wen said.\nIn addition to being more convenient, the most logical return-to-work testing strategy is using rapid antigen tests, Ko said.\n“Just as with any test for anything, if you use it for the wrong purpose, you’re going to get an inappropriate result,” Wen added. However, “if somebody has had multiple antigen tests that are negative, but they have symptoms of Covid, having a PCR test can help to confirm if they have it or not.”\nSome places, such as\nAntarctica\nand\nKosovo\n, require travelers to show proof of a negative PCR test before entering. “If the requirement is a PCR within like 72 hours of travel or similar, a lot can happen in those three days,” Somers said. “That negative PCR from a specimen that was taken three days before travel may not be any better than a rapid on the day of travel.\n“For the issue of somebody who had an infection and is recovering, I think this is where it’s really an administrative decision in terms of how much risk the country is willing to take (or) how much margin of error,” she added. “If a country is trying to be extremely strict in terms of keeping out all infections, then having a negative PCR will be a more sure way of doing that. However, it’s going to potentially prolong the amount of time between when somebody can enter the country compared to when they were infectious.”\nCNN’s Kaitlan Collins, John Bonifield, Nikki Carvajal, Eric Levenson and Arman Azad contributed to this story.","attrs_markdown":"Thank You\\!\n\nYour effort and contribution in providing this feedback is much appreciated.\n\nClose\n\nAd Feedback\n\n[Health](https:\/\/www.cnn.com\/health)\n\n[Life, But Better](https:\/\/www.cnn.com\/interactive\/life-but-better\/)\n\n[Fitness](https:\/\/www.cnn.com\/health\/life-but-better\/fitness)\n\n[Food](http:\/\/www.cnn.com\/health\/life-but-better\/food)\n\n[Sleep](https:\/\/www.cnn.com\/health\/life-but-better\/sleep)\n\n[Mindfulness](https:\/\/www.cnn.com\/health\/life-but-better\/mindfulness)\n\n[Relationships](https:\/\/www.cnn.com\/health\/life-but-better\/relationships)\n\n[More]()\n\n[Life, But Better](https:\/\/www.cnn.com\/interactive\/life-but-better\/)   [Fitness](https:\/\/www.cnn.com\/health\/life-but-better\/fitness)   [Food](http:\/\/www.cnn.com\/health\/life-but-better\/food)   [Sleep](https:\/\/www.cnn.com\/health\/life-but-better\/sleep)   [Mindfulness](https:\/\/www.cnn.com\/health\/life-but-better\/mindfulness)   [Relationships](https:\/\/www.cnn.com\/health\/life-but-better\/relationships)\n\n[Watch](https:\/\/www.cnn.com\/subscription\/all-access)   [Listen](https:\/\/www.cnn.com\/audio)\n\nSubscribe\n\nSign in\n\nMy Account\n\n- [Settings](https:\/\/www.cnn.com\/account\/settings)\n- [Newsletters](https:\/\/www.cnn.com\/newsletters)\n- [Topics you follow](https:\/\/www.cnn.com\/follow?iid=fw_var-nav)\n- [Sign out](https:\/\/www.cnn.com\/2022\/01\/19\/health\/pcr-vs-antigen-tests-covid-19-wellness)\n\nYour CNN account Sign in to your CNN account\n***\nSign in\n\nMy Account\n\n- [Settings](https:\/\/www.cnn.com\/account\/settings)\n- [Newsletters](https:\/\/www.cnn.com\/newsletters)\n- [Topics you follow](https:\/\/www.cnn.com\/follow?iid=fw_var-nav)\n- [Sign out](https:\/\/www.cnn.com\/2022\/01\/19\/health\/pcr-vs-antigen-tests-covid-19-wellness)\n\nYour CNN account Sign in to your CNN account\n***\n[Listen](https:\/\/www.cnn.com\/audio)   [Watch](https:\/\/www.cnn.com\/subscription\/all-access)\n***\nEdition\n\n[US](https:\/\/us.cnn.com\/?hpt=header_edition-picker)\n\n[International](https:\/\/edition.cnn.com\/?hpt=header_edition-picker)\n\n[Arabic](https:\/\/arabic.cnn.com\/?hpt=header_edition-picker)\n\n[Español](https:\/\/cnnespanol.cnn.com\/?hpt=header_edition-picker)\n\nEdition\n\n- [US](https:\/\/us.cnn.com\/?hpt=header_edition-picker)\n- [International](https:\/\/edition.cnn.com\/?hpt=header_edition-picker)\n- [Arabic](https:\/\/arabic.cnn.com\/?hpt=header_edition-picker)\n- [Español](https:\/\/cnnespanol.cnn.com\/?hpt=header_edition-picker)\n***\n[Life, But Better](https:\/\/www.cnn.com\/interactive\/life-but-better\/)\n\n[Fitness](https:\/\/www.cnn.com\/health\/life-but-better\/fitness)\n\n[Food](http:\/\/www.cnn.com\/health\/life-but-better\/food)\n\n[Sleep](https:\/\/www.cnn.com\/health\/life-but-better\/sleep)\n\n[Mindfulness](https:\/\/www.cnn.com\/health\/life-but-better\/mindfulness)\n\n[Relationships](https:\/\/www.cnn.com\/health\/life-but-better\/relationships)\n***\nFollow CNN\n\nDownload the CNN App\n***\n\n- [US](https:\/\/www.cnn.com\/us)\n  - 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Watch live](https:\/\/www.cnn.com\/live-video-2)\n\n[![Life, But Better - Other](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)](https:\/\/www.cnn.com\/specials\/health\/life-but-better)\n\nAd Feedback\n\n# When you should take a PCR vs. a rapid antigen test\nBy [Kristen Rogers](https:\/\/www.cnn.com\/profiles\/kristen-rogers), CNN\n\n6 min read\n\nUpdated 7:54 PM EST, Wed January 19, 2022\n\nLink Copied\\!\n\n**Unable to load video**\n\nPlease try again later.\n\nError occurred while fetching labs\n\nSubtitles\n\n- Off\n\nVideo Ad Feedback\n\nHow to get your at-home Covid-19 test for free\n\n02:37   • Source: [CNN](https:\/\/www.cnn.com\/)\n\nCoronavirus  16 videos\n\n![brian china economic crisis wang 0715](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\n'I'm losing my mind': Chinese depositor loses life savings in bank\n\n03:58\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![LOS ANGELES, CALIFORNIA - APRIL 21: A man is asked to cough into his arm as part of testing for COVID-19, by a member of the Los Angeles Fire Department wearing personal protective equipment (PPE) in Skid Row, amidst the coronavirus pandemic on April 21, 2020 in Los Angeles, California. 43 people tested positive for COVID-19 at one nearby Skid Row homeless shelter. (Photo by Mario Tama\/Getty Images)](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nDoctor: Covid-19 BA.5 variant is highly transmissible\n\n02:31\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![Fauci](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nDr. Fauci explains why new BA.5 subvariant is concerning\n\n04:58\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![FILE - Youngstown City Health Department worker Faith Terreri grabs two at-home COVID-19 test kits to be handed out during a distribution event, Dec. 30, 2021, in Youngstown, Ohio. Starting Saturday, private health insurers will be required to cover up to eight home COVID-19 tests per month for those on their plans, the Biden administration announced Monday, as it looks to lower costs and make testing for the virus more convenient amid rising frustrations. (AP Photo\/David Dermer, File)](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nHow to get your at-home Covid-19 test for free\n\n02:37\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![gupta screengrab vpx](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nWill approaching winter mean new Covid-19 surge?\n\n04:20\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![china corona nyc](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nArtist wears 27 hazmat suits to protest China's policies\n\n03:08\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![hong kong tourism](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nSee how Hong Kong is trying to revitalize itself after Covid-19\n\n02:23\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![Late night Joe Biden](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nSee late night reactions to President Biden claiming pandemic over\n\n01:45\n\nNow playing\n\n• Source: CNN Business\n\n![Patient using video game treatment long haul covid gupta pkg vpx](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nDr. Gupta: This treatment for 'brain fog' from long-Covid just might surprise you\n\n04:05\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![joe rogan aaron rogers covid vaccination orig mg\\_00000000.png](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nAaron Rodgers tells Joe Rogan why he didn't get vaccinated for Covid\n\n02:16\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![BARTLETT, ILLINOIS - MAY 01: Seven-year-old Hamza Haqqani, a 2nd grade student at Al-Huda Academy, uses a computer to participate in an E-learning class with his teacher and classmates while at his home on May 01, 2020 in Bartlett, Illinois. Al-Huda Academy, an Islam based private school that teaches pre-school through the 6th grade students, has had to adopt an E-learning program to finish the school year after all schools in the state were forced to cancel classes in an attempt to curtail the spread of the COVID-19 pandemic. (Photo by Scott Olson\/Getty Images)](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nHow Covid upended children's lives\n\n10:16\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\nVideo Ad Feedback\n\n'Wasn't too surprising': Doctor reacts to Biden's positive Covid test\n\n02:50\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![video thumbnail wuhan market 1](https:\/\/media.cnn.com\/api\/v1\/images\/stellar\/prod\/220727082531-video-thumbnail-wuhan-market-1.jpg?c=16x9&q=w_250,c_fill)\n\nVideo Ad Feedback\n\nNew studies suggest Covid-19 likely originated from Wuhan wet market\n\n01:39\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![Dr. Anthony Fauci](https:\/\/media.cnn.com\/api\/v1\/images\/stellar\/prod\/220723084617-dr-anthony-fauci.jpg?c=16x9&q=w_250,c_fill)\n\nVideo Ad Feedback\n\nDr. Fauci gives update on Biden's Covid-19 infection\n\n05:50\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![doctor jonathan reiner](https:\/\/media.cnn.com\/api\/v1\/images\/stellar\/prod\/220721112856-doctor-jonathan-reiner.jpg?c=16x9&q=w_250,c_fill)\n\nVideo Ad Feedback\n\n'This was entirely predictable': Doctor on Biden testing positive for Covid-19\n\n01:22\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![Jill Biden](https:\/\/media.cnn.com\/api\/v1\/images\/stellar\/prod\/220721110825-jill-biden.jpg?c=16x9&q=w_250,c_fill)\n\nVideo Ad Feedback\n\nJill Biden speaks out after President Biden contracts Covid-19\n\n00:45\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![brian china economic crisis wang 0715](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\n'I'm losing my mind': Chinese depositor loses life savings in bank\n\n03:58\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![LOS ANGELES, CALIFORNIA - APRIL 21: A man is asked to cough into his arm as part of testing for COVID-19, by a member of the Los Angeles Fire Department wearing personal protective equipment (PPE) in Skid Row, amidst the coronavirus pandemic on April 21, 2020 in Los Angeles, California. 43 people tested positive for COVID-19 at one nearby Skid Row homeless shelter. (Photo by Mario Tama\/Getty Images)](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nDoctor: Covid-19 BA.5 variant is highly transmissible\n\n02:31\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![Fauci](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nDr. Fauci explains why new BA.5 subvariant is concerning\n\n04:58\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![FILE - Youngstown City Health Department worker Faith Terreri grabs two at-home COVID-19 test kits to be handed out during a distribution event, Dec. 30, 2021, in Youngstown, Ohio. Starting Saturday, private health insurers will be required to cover up to eight home COVID-19 tests per month for those on their plans, the Biden administration announced Monday, as it looks to lower costs and make testing for the virus more convenient amid rising frustrations. (AP Photo\/David Dermer, File)](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nHow to get your at-home Covid-19 test for free\n\n02:37\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![gupta screengrab vpx](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nWill approaching winter mean new Covid-19 surge?\n\n04:20\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\n![china corona nyc](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nVideo Ad Feedback\n\nArtist wears 27 hazmat suits to protest China's policies\n\n03:08\n\nNow playing\n\n• Source: [CNN](https:\/\/www.cnn.com\/)\n\nSee more videos\n\nCNN\n\n—\n\nA PCR test is often considered the gold standard for detecting coronavirus infection. It’s the test some employers want employees to take before returning to work after having Covid-19, and the one some people scramble to schedule to know whether they can travel or stop isolating. But experts say the PCR test isn’t the best one for every situation.\n\nPCR – or reverse transcription-polymerase chain reaction – tests [can detect small amounts](https:\/\/www.sciencedirect.com\/topics\/neuroscience\/nucleic-acid-amplification) of the coronavirus’ genetic material in a specimen collected from a human. The test then works by amplifying, or making copies of, that genetic material if any is present in a person’s sample, according to the US Centers for Disease Control and Prevention.\n\nThe test’s amplifying property enables it to detect very small amounts of coronavirus in a specimen, “making these tests highly sensitive for diagnosing COVID-19,” according to [the CDC](https:\/\/www.cdc.gov\/coronavirus\/2019-ncov\/lab\/naats.html#:~:text=The%20NAAT%20procedure%20works%20by,sensitive%20for%20diagnosing%20COVID-19.). While this sensitivity can be advantageous for detecting coronavirus after a recent exposure, it also means PCR results can be positive even after you’re no longer contagious.\n\nAd Feedback\n\nAd Feedback\n\n![A worker at a drive-up coronavirus testing clinic administers PCR coronavirus tests in Puyallup, Washington state, on January 4. ](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nA worker at a drive-up coronavirus testing clinic administers PCR coronavirus tests in Puyallup, Washington state, on January 4.\n\nTed S. Warren\/AP\/FILE\n\nA PCR test might say you’re positive for coronavirus for three or four weeks after you’ve recovered because it’s still “picking up past infection and the small fragments (of the virus) are still being amplified,” said CNN Medical Analyst Dr. Leana Wen, an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health. After seven to 10 days, “that PCR test is not an appropriate test.”\n\nRegardless of your situation, here’s what you need to know about the differences between PCR and antigen tests (called [rapid lateral flow tests](https:\/\/www.nhs.uk\/conditions\/coronavirus-covid-19\/testing\/regular-rapid-coronavirus-tests-if-you-do-not-have-symptoms\/) in the UK) and when you should use them.\n\n## When a PCR test is key\nThe prime time to take a PCR test is when you have had a known or suspected exposure to someone with Covid-19 or are experiencing symptoms, and you want to find out if you have a coronavirus infection, said Dr. Albert Ko, the Raj and Indra Nooyi Professor of Public Health at the Yale School of Public Health.\n\nKnowing your Covid-19 status as early in an infection as possible can help you figure out whether you’re infectious to other people, what to tell recent close contacts for their own safety, and what to share with your doctor so they can talk with you about your symptoms and prescribe any medical care if needed, said Emily Somers, an epidemiologist who holds professorships in internal medicine, environmental health sciences, and obstetrics and gynecology at the University of Michigan’s schools of medicine and public health.\n\n[![A Covid-19 test, received through a government program, at a home in Easton, New Hampshire, in December 2021.](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg) A Covid-19 test, received through a government program, at a home in Easton, New Hampshire, in December 2021. John Tully\/Bloomberg\/Getty Images Related article How to get free at-home Covid-19 tests from the government](https:\/\/www.cnn.com\/2022\/01\/19\/politics\/free-government-home-covid-tests-wellness\/index.html)\n\nThat’s important because “up to two days before symptoms, or the one day or two days after symptoms (begin) people can have the highest risk of infecting others,” Ko said. When people are first exposed, it takes time for the virus to replicate enough for a test to pick up on it, Ko explained. PCR tests detect it earlier in the course of infection after exposure than the less sensitive rapid antigen test, which works by detecting a specific protein on one of the coronavirus’ spikes.\n\nThe US Food and Drug Administration has authorized use of both PCR tests and [rapid antigen tests](https:\/\/www.cdc.gov\/coronavirus\/2019-ncov\/lab\/resources\/antigen-tests-guidelines.html) in laboratory-based or point-of-care settings (such as a physician’s office, urgent care facility, pharmacy, school health clinic and temporary locations like drive-through testing sites). Antigen tests are also commonly available as self-tests.\n\n## When rapid antigen tests are crucial\nContrary to what some still think is best, a rapid antigen test (or rapid lateral flow test) is what you should be taking after you have had Covid-19 for several days and want to confirm you’re probably no longer infectious to other people. “‘Infected’ means that I have the virus in me,” Ko explained. “’Infectious’ means that I’m shedding enough virus to infect somebody else, and that only occurs during a peak. It follows a curve after someone has been exposed.”\n\n[![Dr. Sanjay Gupta Holiday tips](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg) video Related video Watch Dr. Gupta take an at-home Covid test](https:\/\/www.cnn.com\/videos\/health\/2021\/12\/17\/sanjay-gupta-covid-test-holidays.cnn)\n\nThe CDC’s [current isolation guidance](https:\/\/www.cnn.com\/2022\/01\/04\/health\/cdc-updated-guidance-covid-isolation\/index.html) is that if people have access to a coronavirus test and want to take it, the best approach is using a rapid test toward the end of their five-day isolation period. Those who continue to test positive at that point should continue isolating until they reach 10 days after their symptoms began. People who test negative can stop isolating but should wear a mask around others until day 10.\n\n![Aaron Salvador swabs his nose with a coronavirus rapid antigen test kit in Washington, DC, on December 29, 2021.](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg)\n\nAaron Salvador swabs his nose with a coronavirus rapid antigen test kit in Washington, DC, on December 29, 2021.\n\nTom Williams\/CQ-Roll Call, Inc.\/Getty Images\/FILE\n\n“Even though the rapid (antigen) test has low sensitivity and is inferior to the PCR test to tell you if you’ve been infected, because of its lower sensitivity, it picks up only viruses at a higher level and probably levels that are more infectious,” Ko explained.\n\n“At this point, we don’t have an FDA-cleared test that can tell us exactly” how much virus is needed to be infectious to other people, Ko added. But evolving evidence has led public health experts to think that rapid tests do better than PCR tests at capturing when people are exiting the period of maximum infectiousness, since PCR tests are too sensitive, he said.\n\n“We have other data to indicate at what point somebody is no longer infectious,” Wen said. A [UK preprint study](https:\/\/www.medrxiv.org\/content\/10.1101\/2021.12.23.21268326v1.full.pdf) published in December 2021 found that after five days of coronavirus infection, 31% of people were still infectious. On day eight, that percentage dropped to 11% and continued to decrease over the next several days.\n\n“Having the presence of two rapid tests – two days in a row – that are both negative is a very good indicator a person is no longer infectious,” Somers said.\n\nThis doesn’t mean that being contagious at this point is totally impossible – it just means that once you have had Covid-19 for several days and then test negative with a rapid antigen test, the viral RNA is likely at levels low enough to not be infectious (even if you still test positive on a PCR test).\n\n[![covid self test wellness RESTRICTED](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn-fallback-image.jpg) John Tully\/Bloomberg\/Getty Images Related article Going somewhere? Here's how to test yourself for Covid-19 before you do](https:\/\/www.cnn.com\/2021\/12\/17\/health\/how-to-at-home-covid-19-test-wellness\/index.html)\n\nThere have been many online reports of employers requiring people to show proof of a negative PCR test before they return to work, which is “probably not coherent thinking” and has people not returning to work until weeks after their initial rapid antigen or PCR test, Ko said.\n\n“I would highly urge individuals to contact their HR (human resources department) – or if there’s a medical or Covid lead on their teams – and escalate the issue because it’s not reasonable,” Wen said.\n\nIn addition to being more convenient, the most logical return-to-work testing strategy is using rapid antigen tests, Ko said.\n\n“Just as with any test for anything, if you use it for the wrong purpose, you’re going to get an inappropriate result,” Wen added. However, “if somebody has had multiple antigen tests that are negative, but they have symptoms of Covid, having a PCR test can help to confirm if they have it or not.”\n\n### Get CNN Health's weekly newsletter\n[Sign up here to get **The Results Are In with Dr. Sanjay Gupta**](https:\/\/form.cnn.com\/results_are_in?source=article) every Tuesday from the CNN Health team.\n\nSome places, such as [Antarctica](https:\/\/www.cnn.com\/travel\/article\/antarctica-travel-covid-19\/index.html) and [Kosovo](https:\/\/xk.usembassy.gov\/coronavirus\/), require travelers to show proof of a negative PCR test before entering. “If the requirement is a PCR within like 72 hours of travel or similar, a lot can happen in those three days,” Somers said. “That negative PCR from a specimen that was taken three days before travel may not be any better than a rapid on the day of travel.\n\n“For the issue of somebody who had an infection and is recovering, I think this is where it’s really an administrative decision in terms of how much risk the country is willing to take (or) how much margin of error,” she added. “If a country is trying to be extremely strict in terms of keeping out all infections, then having a negative PCR will be a more sure way of doing that. However, it’s going to potentially prolong the amount of time between when somebody can enter the country compared to when they were infectious.”\n\n*CNN’s Kaitlan Collins, John Bonifield, Nikki Carvajal, Eric Levenson and Arman Azad contributed to this story.*\n\nAd Feedback\n\nAd Feedback\n\nAd Feedback\n\nSubscribe\n\nSign in\n\nMy Account\n\n- [Settings](https:\/\/www.cnn.com\/account\/settings)\n- [Newsletters](https:\/\/www.cnn.com\/newsletters)\n- [Topics you follow](https:\/\/www.cnn.com\/follow?iid=fw_var-nav)\n- [Sign out](https:\/\/www.cnn.com\/2022\/01\/19\/health\/pcr-vs-antigen-tests-covid-19-wellness)\n\nYour CNN account Sign in to your CNN account\n***\n[Listen](https:\/\/www.cnn.com\/audio)   [Watch](https:\/\/www.cnn.com\/watch)\n***\n- [US](https:\/\/www.cnn.com\/us)\n  - [Crime & Justice](https:\/\/www.cnn.com\/us\/crime-and-justice)\n  - [Immigration](https:\/\/www.cnn.com\/us\/immigration)\n  - [Education](https:\/\/www.cnn.com\/us\/education)\n  - [Transportation](https:\/\/www.cnn.com\/us\/transportation)\n  - 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A Warner Bros. Discovery Company. All Rights Reserved.   \n CNN Sans ™ & © 2016 Cable News Network.\n\n## Download the CNN app\nScan the QR code to download the CNN app on Google Play.\n\n![Scan the QR code to download the CNN app from Google Play.](https:\/\/www.cnn.com\/media\/sites\/cnn\/google-play-cnn-app-qr-code.png)\n\n## Download the CNN app\nScan the QR code to download the CNN app from the Apple Store.\n\n![Scan the QR code to download the CNN app from the Apple Store.](https:\/\/www.cnn.com\/media\/sites\/cnn\/app-store-cnn-app-qr-code.png)\n\n![CNN Logo](https:\/\/www.cnn.com\/media\/sites\/cnn\/cnn.svg)\n\n## Legal Terms and Privacy\nBy clicking Agree, you have read and agree to the Terms of Use and agree to the collection and use of your information by cookies and similar technologies, as set forth in our Privacy Policy.\n\nBy clicking \"Agree\", you have read and agree to the [Terms of Use](https:\/\/www.cnn.com\/terms) and agree to the collection and use of your information by cookies and similar technologies, as set forth in our [Privacy Policy](https:\/\/www.cnn.com\/privacy).\n\n[Agree](https:\/\/www.cnn.com\/2022\/01\/19\/health\/pcr-vs-antigen-tests-covid-19-wellness)","attrs_readable_markdown":"CNN —\n\nA PCR test is often considered the gold standard for detecting coronavirus infection. It’s the test some employers want employees to take before returning to work after having Covid-19, and the one some people scramble to schedule to know whether they can travel or stop isolating. But experts say the PCR test isn’t the best one for every situation.\n\nPCR – or reverse transcription-polymerase chain reaction – tests [can detect small amounts](https:\/\/www.sciencedirect.com\/topics\/neuroscience\/nucleic-acid-amplification) of the coronavirus’ genetic material in a specimen collected from a human. The test then works by amplifying, or making copies of, that genetic material if any is present in a person’s sample, according to the US Centers for Disease Control and Prevention.\n\nThe test’s amplifying property enables it to detect very small amounts of coronavirus in a specimen, “making these tests highly sensitive for diagnosing COVID-19,” according to [the CDC](https:\/\/www.cdc.gov\/coronavirus\/2019-ncov\/lab\/naats.html#:~:text=The%20NAAT%20procedure%20works%20by,sensitive%20for%20diagnosing%20COVID-19.). While this sensitivity can be advantageous for detecting coronavirus after a recent exposure, it also means PCR results can be positive even after you’re no longer contagious.\n\nAd Feedback\n\nAd Feedback\n\nA PCR test might say you’re positive for coronavirus for three or four weeks after you’ve recovered because it’s still “picking up past infection and the small fragments (of the virus) are still being amplified,” said CNN Medical Analyst Dr. Leana Wen, an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health. After seven to 10 days, “that PCR test is not an appropriate test.”\n\nRegardless of your situation, here’s what you need to know about the differences between PCR and antigen tests (called [rapid lateral flow tests](https:\/\/www.nhs.uk\/conditions\/coronavirus-covid-19\/testing\/regular-rapid-coronavirus-tests-if-you-do-not-have-symptoms\/) in the UK) and when you should use them.\n\nThe prime time to take a PCR test is when you have had a known or suspected exposure to someone with Covid-19 or are experiencing symptoms, and you want to find out if you have a coronavirus infection, said Dr. Albert Ko, the Raj and Indra Nooyi Professor of Public Health at the Yale School of Public Health.\n\nKnowing your Covid-19 status as early in an infection as possible can help you figure out whether you’re infectious to other people, what to tell recent close contacts for their own safety, and what to share with your doctor so they can talk with you about your symptoms and prescribe any medical care if needed, said Emily Somers, an epidemiologist who holds professorships in internal medicine, environmental health sciences, and obstetrics and gynecology at the University of Michigan’s schools of medicine and public health.\n\nThat’s important because “up to two days before symptoms, or the one day or two days after symptoms (begin) people can have the highest risk of infecting others,” Ko said. When people are first exposed, it takes time for the virus to replicate enough for a test to pick up on it, Ko explained. PCR tests detect it earlier in the course of infection after exposure than the less sensitive rapid antigen test, which works by detecting a specific protein on one of the coronavirus’ spikes.\n\nThe US Food and Drug Administration has authorized use of both PCR tests and [rapid antigen tests](https:\/\/www.cdc.gov\/coronavirus\/2019-ncov\/lab\/resources\/antigen-tests-guidelines.html) in laboratory-based or point-of-care settings (such as a physician’s office, urgent care facility, pharmacy, school health clinic and temporary locations like drive-through testing sites). Antigen tests are also commonly available as self-tests.\n\nContrary to what some still think is best, a rapid antigen test (or rapid lateral flow test) is what you should be taking after you have had Covid-19 for several days and want to confirm you’re probably no longer infectious to other people. “‘Infected’ means that I have the virus in me,” Ko explained. “’Infectious’ means that I’m shedding enough virus to infect somebody else, and that only occurs during a peak. It follows a curve after someone has been exposed.”\n\nThe CDC’s [current isolation guidance](https:\/\/www.cnn.com\/2022\/01\/04\/health\/cdc-updated-guidance-covid-isolation\/index.html) is that if people have access to a coronavirus test and want to take it, the best approach is using a rapid test toward the end of their five-day isolation period. Those who continue to test positive at that point should continue isolating until they reach 10 days after their symptoms began. People who test negative can stop isolating but should wear a mask around others until day 10.\n\n“Even though the rapid (antigen) test has low sensitivity and is inferior to the PCR test to tell you if you’ve been infected, because of its lower sensitivity, it picks up only viruses at a higher level and probably levels that are more infectious,” Ko explained.\n\n“At this point, we don’t have an FDA-cleared test that can tell us exactly” how much virus is needed to be infectious to other people, Ko added. But evolving evidence has led public health experts to think that rapid tests do better than PCR tests at capturing when people are exiting the period of maximum infectiousness, since PCR tests are too sensitive, he said.\n\n“We have other data to indicate at what point somebody is no longer infectious,” Wen said. A [UK preprint study](https:\/\/www.medrxiv.org\/content\/10.1101\/2021.12.23.21268326v1.full.pdf) published in December 2021 found that after five days of coronavirus infection, 31% of people were still infectious. On day eight, that percentage dropped to 11% and continued to decrease over the next several days.\n\n“Having the presence of two rapid tests – two days in a row – that are both negative is a very good indicator a person is no longer infectious,” Somers said.\n\nThis doesn’t mean that being contagious at this point is totally impossible – it just means that once you have had Covid-19 for several days and then test negative with a rapid antigen test, the viral RNA is likely at levels low enough to not be infectious (even if you still test positive on a PCR test).\n\nThere have been many online reports of employers requiring people to show proof of a negative PCR test before they return to work, which is “probably not coherent thinking” and has people not returning to work until weeks after their initial rapid antigen or PCR test, Ko said.\n\n“I would highly urge individuals to contact their HR (human resources department) – or if there’s a medical or Covid lead on their teams – and escalate the issue because it’s not reasonable,” Wen said.\n\nIn addition to being more convenient, the most logical return-to-work testing strategy is using rapid antigen tests, Ko said.\n\n“Just as with any test for anything, if you use it for the wrong purpose, you’re going to get an inappropriate result,” Wen added. However, “if somebody has had multiple antigen tests that are negative, but they have symptoms of Covid, having a PCR test can help to confirm if they have it or not.”\n\nSome places, such as [Antarctica](https:\/\/www.cnn.com\/travel\/article\/antarctica-travel-covid-19\/index.html) and [Kosovo](https:\/\/xk.usembassy.gov\/coronavirus\/), require travelers to show proof of a negative PCR test before entering. “If the requirement is a PCR within like 72 hours of travel or similar, a lot can happen in those three days,” Somers said. “That negative PCR from a specimen that was taken three days before travel may not be any better than a rapid on the day of travel.\n\n“For the issue of somebody who had an infection and is recovering, I think this is where it’s really an administrative decision in terms of how much risk the country is willing to take (or) how much margin of error,” she added. “If a country is trying to be extremely strict in terms of keeping out all infections, then having a negative PCR will be a more sure way of doing that. However, it’s going to potentially prolong the amount of time between when somebody can enter the country compared to when they were infectious.”\n\n*CNN’s Kaitlan Collins, John Bonifield, Nikki Carvajal, Eric Levenson and Arman Azad contributed to this story.*","meta_canonical":null}

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URL	https://www.cnn.com/2022/01/19/health/pcr-vs-antigen-tests-covid-19-wellness
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Meta Title	PCR vs. antigen tests: Which you should take after having Covid-19 \| CNN
Meta Description	A PCR test is considered the gold standard for detecting coronavirus infection after exposure to Covid-19 or while you’re experiencing symptoms. Coronavirus test experts explain why the rapid antigen test is best for knowing if you’re still infectious.
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Boilerpipe Text	CNN — A PCR test is often considered the gold standard for detecting coronavirus infection. It’s the test some employers want employees to take before returning to work after having Covid-19, and the one some people scramble to schedule to know whether they can travel or stop isolating. But experts say the PCR test isn’t the best one for every situation. PCR – or reverse transcription-polymerase chain reaction – tests can detect small amounts of the coronavirus’ genetic material in a specimen collected from a human. The test then works by amplifying, or making copies of, that genetic material if any is present in a person’s sample, according to the US Centers for Disease Control and Prevention. The test’s amplifying property enables it to detect very small amounts of coronavirus in a specimen, “making these tests highly sensitive for diagnosing COVID-19,” according to the CDC . While this sensitivity can be advantageous for detecting coronavirus after a recent exposure, it also means PCR results can be positive even after you’re no longer contagious. Ad Feedback Ad Feedback A PCR test might say you’re positive for coronavirus for three or four weeks after you’ve recovered because it’s still “picking up past infection and the small fragments (of the virus) are still being amplified,” said CNN Medical Analyst Dr. Leana Wen, an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health. After seven to 10 days, “that PCR test is not an appropriate test.” Regardless of your situation, here’s what you need to know about the differences between PCR and antigen tests (called rapid lateral flow tests in the UK) and when you should use them. The prime time to take a PCR test is when you have had a known or suspected exposure to someone with Covid-19 or are experiencing symptoms, and you want to find out if you have a coronavirus infection, said Dr. Albert Ko, the Raj and Indra Nooyi Professor of Public Health at the Yale School of Public Health. Knowing your Covid-19 status as early in an infection as possible can help you figure out whether you’re infectious to other people, what to tell recent close contacts for their own safety, and what to share with your doctor so they can talk with you about your symptoms and prescribe any medical care if needed, said Emily Somers, an epidemiologist who holds professorships in internal medicine, environmental health sciences, and obstetrics and gynecology at the University of Michigan’s schools of medicine and public health. That’s important because “up to two days before symptoms, or the one day or two days after symptoms (begin) people can have the highest risk of infecting others,” Ko said. When people are first exposed, it takes time for the virus to replicate enough for a test to pick up on it, Ko explained. PCR tests detect it earlier in the course of infection after exposure than the less sensitive rapid antigen test, which works by detecting a specific protein on one of the coronavirus’ spikes. The US Food and Drug Administration has authorized use of both PCR tests and rapid antigen tests in laboratory-based or point-of-care settings (such as a physician’s office, urgent care facility, pharmacy, school health clinic and temporary locations like drive-through testing sites). Antigen tests are also commonly available as self-tests. Contrary to what some still think is best, a rapid antigen test (or rapid lateral flow test) is what you should be taking after you have had Covid-19 for several days and want to confirm you’re probably no longer infectious to other people. “‘Infected’ means that I have the virus in me,” Ko explained. “’Infectious’ means that I’m shedding enough virus to infect somebody else, and that only occurs during a peak. It follows a curve after someone has been exposed.” The CDC’s current isolation guidance is that if people have access to a coronavirus test and want to take it, the best approach is using a rapid test toward the end of their five-day isolation period. Those who continue to test positive at that point should continue isolating until they reach 10 days after their symptoms began. People who test negative can stop isolating but should wear a mask around others until day 10. “Even though the rapid (antigen) test has low sensitivity and is inferior to the PCR test to tell you if you’ve been infected, because of its lower sensitivity, it picks up only viruses at a higher level and probably levels that are more infectious,” Ko explained. “At this point, we don’t have an FDA-cleared test that can tell us exactly” how much virus is needed to be infectious to other people, Ko added. But evolving evidence has led public health experts to think that rapid tests do better than PCR tests at capturing when people are exiting the period of maximum infectiousness, since PCR tests are too sensitive, he said. “We have other data to indicate at what point somebody is no longer infectious,” Wen said. A UK preprint study published in December 2021 found that after five days of coronavirus infection, 31% of people were still infectious. On day eight, that percentage dropped to 11% and continued to decrease over the next several days. “Having the presence of two rapid tests – two days in a row – that are both negative is a very good indicator a person is no longer infectious,” Somers said. This doesn’t mean that being contagious at this point is totally impossible – it just means that once you have had Covid-19 for several days and then test negative with a rapid antigen test, the viral RNA is likely at levels low enough to not be infectious (even if you still test positive on a PCR test). There have been many online reports of employers requiring people to show proof of a negative PCR test before they return to work, which is “probably not coherent thinking” and has people not returning to work until weeks after their initial rapid antigen or PCR test, Ko said. “I would highly urge individuals to contact their HR (human resources department) – or if there’s a medical or Covid lead on their teams – and escalate the issue because it’s not reasonable,” Wen said. In addition to being more convenient, the most logical return-to-work testing strategy is using rapid antigen tests, Ko said. “Just as with any test for anything, if you use it for the wrong purpose, you’re going to get an inappropriate result,” Wen added. However, “if somebody has had multiple antigen tests that are negative, but they have symptoms of Covid, having a PCR test can help to confirm if they have it or not.” Some places, such as Antarctica and Kosovo , require travelers to show proof of a negative PCR test before entering. “If the requirement is a PCR within like 72 hours of travel or similar, a lot can happen in those three days,” Somers said. “That negative PCR from a specimen that was taken three days before travel may not be any better than a rapid on the day of travel. “For the issue of somebody who had an infection and is recovering, I think this is where it’s really an administrative decision in terms of how much risk the country is willing to take (or) how much margin of error,” she added. “If a country is trying to be extremely strict in terms of keeping out all infections, then having a negative PCR will be a more sure way of doing that. However, it’s going to potentially prolong the amount of time between when somebody can enter the country compared to when they were infectious.” CNN’s Kaitlan Collins, John Bonifield, Nikki Carvajal, Eric Levenson and Arman Azad contributed to this story.
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Watch live](https://www.cnn.com/live-video-2) [![Life, But Better - Other](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg)](https://www.cnn.com/specials/health/life-but-better) Ad Feedback # When you should take a PCR vs. a rapid antigen test By [Kristen Rogers](https://www.cnn.com/profiles/kristen-rogers), CNN 6 min read Updated 7:54 PM EST, Wed January 19, 2022 Link Copied\! Unable to load video Please try again later. Error occurred while fetching labs Subtitles - Off Video Ad Feedback How to get your at-home Covid-19 test for free 02:37 • Source: [CNN](https://www.cnn.com/) Coronavirus 16 videos ![brian china economic crisis wang 0715](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback 'I'm losing my mind': Chinese depositor loses life savings in bank 03:58 Now playing • Source: [CNN](https://www.cnn.com/) ![LOS ANGELES, CALIFORNIA - APRIL 21: A man is asked to cough into his arm as part of testing for COVID-19, by a member of the Los Angeles Fire Department wearing personal protective equipment (PPE) in Skid Row, amidst the coronavirus pandemic on April 21, 2020 in Los Angeles, California. 43 people tested positive for COVID-19 at one nearby Skid Row homeless shelter. (Photo by Mario Tama/Getty Images)](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Doctor: Covid-19 BA.5 variant is highly transmissible 02:31 Now playing • Source: [CNN](https://www.cnn.com/) ![Fauci](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Dr. Fauci explains why new BA.5 subvariant is concerning 04:58 Now playing • Source: [CNN](https://www.cnn.com/) ![FILE - Youngstown City Health Department worker Faith Terreri grabs two at-home COVID-19 test kits to be handed out during a distribution event, Dec. 30, 2021, in Youngstown, Ohio. Starting Saturday, private health insurers will be required to cover up to eight home COVID-19 tests per month for those on their plans, the Biden administration announced Monday, as it looks to lower costs and make testing for the virus more convenient amid rising frustrations. (AP Photo/David Dermer, File)](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback How to get your at-home Covid-19 test for free 02:37 Now playing • Source: [CNN](https://www.cnn.com/) ![gupta screengrab vpx](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Will approaching winter mean new Covid-19 surge? 04:20 Now playing • Source: [CNN](https://www.cnn.com/) ![china corona nyc](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Artist wears 27 hazmat suits to protest China's policies 03:08 Now playing • Source: [CNN](https://www.cnn.com/) ![hong kong tourism](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback See how Hong Kong is trying to revitalize itself after Covid-19 02:23 Now playing • Source: [CNN](https://www.cnn.com/) ![Late night Joe Biden](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback See late night reactions to President Biden claiming pandemic over 01:45 Now playing • Source: CNN Business ![Patient using video game treatment long haul covid gupta pkg vpx](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Dr. Gupta: This treatment for 'brain fog' from long-Covid just might surprise you 04:05 Now playing • Source: [CNN](https://www.cnn.com/) ![joe rogan aaron rogers covid vaccination orig mg\_00000000.png](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Aaron Rodgers tells Joe Rogan why he didn't get vaccinated for Covid 02:16 Now playing • Source: [CNN](https://www.cnn.com/) ![BARTLETT, ILLINOIS - MAY 01: Seven-year-old Hamza Haqqani, a 2nd grade student at Al-Huda Academy, uses a computer to participate in an E-learning class with his teacher and classmates while at his home on May 01, 2020 in Bartlett, Illinois. Al-Huda Academy, an Islam based private school that teaches pre-school through the 6th grade students, has had to adopt an E-learning program to finish the school year after all schools in the state were forced to cancel classes in an attempt to curtail the spread of the COVID-19 pandemic. (Photo by Scott Olson/Getty Images)](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback How Covid upended children's lives 10:16 Now playing • Source: [CNN](https://www.cnn.com/) Video Ad Feedback 'Wasn't too surprising': Doctor reacts to Biden's positive Covid test 02:50 Now playing • Source: [CNN](https://www.cnn.com/) ![video thumbnail wuhan market 1](https://media.cnn.com/api/v1/images/stellar/prod/220727082531-video-thumbnail-wuhan-market-1.jpg?c=16x9&q=w_250,c_fill) Video Ad Feedback New studies suggest Covid-19 likely originated from Wuhan wet market 01:39 Now playing • Source: [CNN](https://www.cnn.com/) ![Dr. Anthony Fauci](https://media.cnn.com/api/v1/images/stellar/prod/220723084617-dr-anthony-fauci.jpg?c=16x9&q=w_250,c_fill) Video Ad Feedback Dr. Fauci gives update on Biden's Covid-19 infection 05:50 Now playing • Source: [CNN](https://www.cnn.com/) ![doctor jonathan reiner](https://media.cnn.com/api/v1/images/stellar/prod/220721112856-doctor-jonathan-reiner.jpg?c=16x9&q=w_250,c_fill) Video Ad Feedback 'This was entirely predictable': Doctor on Biden testing positive for Covid-19 01:22 Now playing • Source: [CNN](https://www.cnn.com/) ![Jill Biden](https://media.cnn.com/api/v1/images/stellar/prod/220721110825-jill-biden.jpg?c=16x9&q=w_250,c_fill) Video Ad Feedback Jill Biden speaks out after President Biden contracts Covid-19 00:45 Now playing • Source: [CNN](https://www.cnn.com/) ![brian china economic crisis wang 0715](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback 'I'm losing my mind': Chinese depositor loses life savings in bank 03:58 Now playing • Source: [CNN](https://www.cnn.com/) ![LOS ANGELES, CALIFORNIA - APRIL 21: A man is asked to cough into his arm as part of testing for COVID-19, by a member of the Los Angeles Fire Department wearing personal protective equipment (PPE) in Skid Row, amidst the coronavirus pandemic on April 21, 2020 in Los Angeles, California. 43 people tested positive for COVID-19 at one nearby Skid Row homeless shelter. (Photo by Mario Tama/Getty Images)](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Doctor: Covid-19 BA.5 variant is highly transmissible 02:31 Now playing • Source: [CNN](https://www.cnn.com/) ![Fauci](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Dr. Fauci explains why new BA.5 subvariant is concerning 04:58 Now playing • Source: [CNN](https://www.cnn.com/) ![FILE - Youngstown City Health Department worker Faith Terreri grabs two at-home COVID-19 test kits to be handed out during a distribution event, Dec. 30, 2021, in Youngstown, Ohio. Starting Saturday, private health insurers will be required to cover up to eight home COVID-19 tests per month for those on their plans, the Biden administration announced Monday, as it looks to lower costs and make testing for the virus more convenient amid rising frustrations. (AP Photo/David Dermer, File)](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback How to get your at-home Covid-19 test for free 02:37 Now playing • Source: [CNN](https://www.cnn.com/) ![gupta screengrab vpx](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Will approaching winter mean new Covid-19 surge? 04:20 Now playing • Source: [CNN](https://www.cnn.com/) ![china corona nyc](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Video Ad Feedback Artist wears 27 hazmat suits to protest China's policies 03:08 Now playing • Source: [CNN](https://www.cnn.com/) See more videos CNN — A PCR test is often considered the gold standard for detecting coronavirus infection. It’s the test some employers want employees to take before returning to work after having Covid-19, and the one some people scramble to schedule to know whether they can travel or stop isolating. But experts say the PCR test isn’t the best one for every situation. PCR – or reverse transcription-polymerase chain reaction – tests [can detect small amounts](https://www.sciencedirect.com/topics/neuroscience/nucleic-acid-amplification) of the coronavirus’ genetic material in a specimen collected from a human. The test then works by amplifying, or making copies of, that genetic material if any is present in a person’s sample, according to the US Centers for Disease Control and Prevention. The test’s amplifying property enables it to detect very small amounts of coronavirus in a specimen, “making these tests highly sensitive for diagnosing COVID-19,” according to [the CDC](https://www.cdc.gov/coronavirus/2019-ncov/lab/naats.html#:~:text=The%20NAAT%20procedure%20works%20by,sensitive%20for%20diagnosing%20COVID-19.). While this sensitivity can be advantageous for detecting coronavirus after a recent exposure, it also means PCR results can be positive even after you’re no longer contagious. Ad Feedback Ad Feedback ![A worker at a drive-up coronavirus testing clinic administers PCR coronavirus tests in Puyallup, Washington state, on January 4. ](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) A worker at a drive-up coronavirus testing clinic administers PCR coronavirus tests in Puyallup, Washington state, on January 4. Ted S. Warren/AP/FILE A PCR test might say you’re positive for coronavirus for three or four weeks after you’ve recovered because it’s still “picking up past infection and the small fragments (of the virus) are still being amplified,” said CNN Medical Analyst Dr. Leana Wen, an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health. After seven to 10 days, “that PCR test is not an appropriate test.” Regardless of your situation, here’s what you need to know about the differences between PCR and antigen tests (called [rapid lateral flow tests](https://www.nhs.uk/conditions/coronavirus-covid-19/testing/regular-rapid-coronavirus-tests-if-you-do-not-have-symptoms/) in the UK) and when you should use them. ## When a PCR test is key The prime time to take a PCR test is when you have had a known or suspected exposure to someone with Covid-19 or are experiencing symptoms, and you want to find out if you have a coronavirus infection, said Dr. Albert Ko, the Raj and Indra Nooyi Professor of Public Health at the Yale School of Public Health. Knowing your Covid-19 status as early in an infection as possible can help you figure out whether you’re infectious to other people, what to tell recent close contacts for their own safety, and what to share with your doctor so they can talk with you about your symptoms and prescribe any medical care if needed, said Emily Somers, an epidemiologist who holds professorships in internal medicine, environmental health sciences, and obstetrics and gynecology at the University of Michigan’s schools of medicine and public health. [![A Covid-19 test, received through a government program, at a home in Easton, New Hampshire, in December 2021.](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) A Covid-19 test, received through a government program, at a home in Easton, New Hampshire, in December 2021. John Tully/Bloomberg/Getty Images Related article How to get free at-home Covid-19 tests from the government](https://www.cnn.com/2022/01/19/politics/free-government-home-covid-tests-wellness/index.html) That’s important because “up to two days before symptoms, or the one day or two days after symptoms (begin) people can have the highest risk of infecting others,” Ko said. When people are first exposed, it takes time for the virus to replicate enough for a test to pick up on it, Ko explained. PCR tests detect it earlier in the course of infection after exposure than the less sensitive rapid antigen test, which works by detecting a specific protein on one of the coronavirus’ spikes. The US Food and Drug Administration has authorized use of both PCR tests and [rapid antigen tests](https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html) in laboratory-based or point-of-care settings (such as a physician’s office, urgent care facility, pharmacy, school health clinic and temporary locations like drive-through testing sites). Antigen tests are also commonly available as self-tests. ## When rapid antigen tests are crucial Contrary to what some still think is best, a rapid antigen test (or rapid lateral flow test) is what you should be taking after you have had Covid-19 for several days and want to confirm you’re probably no longer infectious to other people. “‘Infected’ means that I have the virus in me,” Ko explained. “’Infectious’ means that I’m shedding enough virus to infect somebody else, and that only occurs during a peak. It follows a curve after someone has been exposed.” [![Dr. Sanjay Gupta Holiday tips](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) video Related video Watch Dr. Gupta take an at-home Covid test](https://www.cnn.com/videos/health/2021/12/17/sanjay-gupta-covid-test-holidays.cnn) The CDC’s [current isolation guidance](https://www.cnn.com/2022/01/04/health/cdc-updated-guidance-covid-isolation/index.html) is that if people have access to a coronavirus test and want to take it, the best approach is using a rapid test toward the end of their five-day isolation period. Those who continue to test positive at that point should continue isolating until they reach 10 days after their symptoms began. People who test negative can stop isolating but should wear a mask around others until day 10. ![Aaron Salvador swabs his nose with a coronavirus rapid antigen test kit in Washington, DC, on December 29, 2021.](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) Aaron Salvador swabs his nose with a coronavirus rapid antigen test kit in Washington, DC, on December 29, 2021. Tom Williams/CQ-Roll Call, Inc./Getty Images/FILE “Even though the rapid (antigen) test has low sensitivity and is inferior to the PCR test to tell you if you’ve been infected, because of its lower sensitivity, it picks up only viruses at a higher level and probably levels that are more infectious,” Ko explained. “At this point, we don’t have an FDA-cleared test that can tell us exactly” how much virus is needed to be infectious to other people, Ko added. But evolving evidence has led public health experts to think that rapid tests do better than PCR tests at capturing when people are exiting the period of maximum infectiousness, since PCR tests are too sensitive, he said. “We have other data to indicate at what point somebody is no longer infectious,” Wen said. A [UK preprint study](https://www.medrxiv.org/content/10.1101/2021.12.23.21268326v1.full.pdf) published in December 2021 found that after five days of coronavirus infection, 31% of people were still infectious. On day eight, that percentage dropped to 11% and continued to decrease over the next several days. “Having the presence of two rapid tests – two days in a row – that are both negative is a very good indicator a person is no longer infectious,” Somers said. This doesn’t mean that being contagious at this point is totally impossible – it just means that once you have had Covid-19 for several days and then test negative with a rapid antigen test, the viral RNA is likely at levels low enough to not be infectious (even if you still test positive on a PCR test). [![covid self test wellness RESTRICTED](https://www.cnn.com/media/sites/cnn/cnn-fallback-image.jpg) John Tully/Bloomberg/Getty Images Related article Going somewhere? Here's how to test yourself for Covid-19 before you do](https://www.cnn.com/2021/12/17/health/how-to-at-home-covid-19-test-wellness/index.html) There have been many online reports of employers requiring people to show proof of a negative PCR test before they return to work, which is “probably not coherent thinking” and has people not returning to work until weeks after their initial rapid antigen or PCR test, Ko said. “I would highly urge individuals to contact their HR (human resources department) – or if there’s a medical or Covid lead on their teams – and escalate the issue because it’s not reasonable,” Wen said. In addition to being more convenient, the most logical return-to-work testing strategy is using rapid antigen tests, Ko said. “Just as with any test for anything, if you use it for the wrong purpose, you’re going to get an inappropriate result,” Wen added. However, “if somebody has had multiple antigen tests that are negative, but they have symptoms of Covid, having a PCR test can help to confirm if they have it or not.” ### Get CNN Health's weekly newsletter [Sign up here to get The Results Are In with Dr. Sanjay Gupta*](https://form.cnn.com/results_are_in?source=article) every Tuesday from the CNN Health team. Some places, such as [Antarctica](https://www.cnn.com/travel/article/antarctica-travel-covid-19/index.html) and [Kosovo](https://xk.usembassy.gov/coronavirus/), require travelers to show proof of a negative PCR test before entering. “If the requirement is a PCR within like 72 hours of travel or similar, a lot can happen in those three days,” Somers said. “That negative PCR from a specimen that was taken three days before travel may not be any better than a rapid on the day of travel. “For the issue of somebody who had an infection and is recovering, I think this is where it’s really an administrative decision in terms of how much risk the country is willing to take (or) how much margin of error,” she added. “If a country is trying to be extremely strict in terms of keeping out all infections, then having a negative PCR will be a more sure way of doing that. However, it’s going to potentially prolong the amount of time between when somebody can enter the country compared to when they were infectious.” CNN’s Kaitlan Collins, John Bonifield, Nikki Carvajal, Eric Levenson and Arman Azad contributed to this story.* Ad Feedback Ad Feedback Ad Feedback Subscribe Sign in My Account - [Settings](https://www.cnn.com/account/settings) - [Newsletters](https://www.cnn.com/newsletters) - [Topics you follow](https://www.cnn.com/follow?iid=fw_var-nav) - [Sign out](https://www.cnn.com/2022/01/19/health/pcr-vs-antigen-tests-covid-19-wellness) Your CNN account Sign in to your CNN account * [Listen](https://www.cnn.com/audio) [Watch](https://www.cnn.com/watch) * - [US](https://www.cnn.com/us) - [Crime & Justice](https://www.cnn.com/us/crime-and-justice) - [Immigration](https://www.cnn.com/us/immigration) - [Education](https://www.cnn.com/us/education) - [Transportation](https://www.cnn.com/us/transportation) - [Race & Identity](https://www.cnn.com/us/race-and-identity) - 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Readable Markdown	CNN — A PCR test is often considered the gold standard for detecting coronavirus infection. It’s the test some employers want employees to take before returning to work after having Covid-19, and the one some people scramble to schedule to know whether they can travel or stop isolating. But experts say the PCR test isn’t the best one for every situation. PCR – or reverse transcription-polymerase chain reaction – tests [can detect small amounts](https://www.sciencedirect.com/topics/neuroscience/nucleic-acid-amplification) of the coronavirus’ genetic material in a specimen collected from a human. The test then works by amplifying, or making copies of, that genetic material if any is present in a person’s sample, according to the US Centers for Disease Control and Prevention. The test’s amplifying property enables it to detect very small amounts of coronavirus in a specimen, “making these tests highly sensitive for diagnosing COVID-19,” according to [the CDC](https://www.cdc.gov/coronavirus/2019-ncov/lab/naats.html#:~:text=The%20NAAT%20procedure%20works%20by,sensitive%20for%20diagnosing%20COVID-19.). While this sensitivity can be advantageous for detecting coronavirus after a recent exposure, it also means PCR results can be positive even after you’re no longer contagious. Ad Feedback Ad Feedback A PCR test might say you’re positive for coronavirus for three or four weeks after you’ve recovered because it’s still “picking up past infection and the small fragments (of the virus) are still being amplified,” said CNN Medical Analyst Dr. Leana Wen, an emergency physician and visiting professor of health policy and management at the George Washington University Milken Institute School of Public Health. After seven to 10 days, “that PCR test is not an appropriate test.” Regardless of your situation, here’s what you need to know about the differences between PCR and antigen tests (called [rapid lateral flow tests](https://www.nhs.uk/conditions/coronavirus-covid-19/testing/regular-rapid-coronavirus-tests-if-you-do-not-have-symptoms/) in the UK) and when you should use them. The prime time to take a PCR test is when you have had a known or suspected exposure to someone with Covid-19 or are experiencing symptoms, and you want to find out if you have a coronavirus infection, said Dr. Albert Ko, the Raj and Indra Nooyi Professor of Public Health at the Yale School of Public Health. Knowing your Covid-19 status as early in an infection as possible can help you figure out whether you’re infectious to other people, what to tell recent close contacts for their own safety, and what to share with your doctor so they can talk with you about your symptoms and prescribe any medical care if needed, said Emily Somers, an epidemiologist who holds professorships in internal medicine, environmental health sciences, and obstetrics and gynecology at the University of Michigan’s schools of medicine and public health. That’s important because “up to two days before symptoms, or the one day or two days after symptoms (begin) people can have the highest risk of infecting others,” Ko said. When people are first exposed, it takes time for the virus to replicate enough for a test to pick up on it, Ko explained. PCR tests detect it earlier in the course of infection after exposure than the less sensitive rapid antigen test, which works by detecting a specific protein on one of the coronavirus’ spikes. The US Food and Drug Administration has authorized use of both PCR tests and [rapid antigen tests](https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html) in laboratory-based or point-of-care settings (such as a physician’s office, urgent care facility, pharmacy, school health clinic and temporary locations like drive-through testing sites). Antigen tests are also commonly available as self-tests. Contrary to what some still think is best, a rapid antigen test (or rapid lateral flow test) is what you should be taking after you have had Covid-19 for several days and want to confirm you’re probably no longer infectious to other people. “‘Infected’ means that I have the virus in me,” Ko explained. “’Infectious’ means that I’m shedding enough virus to infect somebody else, and that only occurs during a peak. It follows a curve after someone has been exposed.” The CDC’s [current isolation guidance](https://www.cnn.com/2022/01/04/health/cdc-updated-guidance-covid-isolation/index.html) is that if people have access to a coronavirus test and want to take it, the best approach is using a rapid test toward the end of their five-day isolation period. Those who continue to test positive at that point should continue isolating until they reach 10 days after their symptoms began. People who test negative can stop isolating but should wear a mask around others until day 10. “Even though the rapid (antigen) test has low sensitivity and is inferior to the PCR test to tell you if you’ve been infected, because of its lower sensitivity, it picks up only viruses at a higher level and probably levels that are more infectious,” Ko explained. “At this point, we don’t have an FDA-cleared test that can tell us exactly” how much virus is needed to be infectious to other people, Ko added. But evolving evidence has led public health experts to think that rapid tests do better than PCR tests at capturing when people are exiting the period of maximum infectiousness, since PCR tests are too sensitive, he said. “We have other data to indicate at what point somebody is no longer infectious,” Wen said. A [UK preprint study](https://www.medrxiv.org/content/10.1101/2021.12.23.21268326v1.full.pdf) published in December 2021 found that after five days of coronavirus infection, 31% of people were still infectious. On day eight, that percentage dropped to 11% and continued to decrease over the next several days. “Having the presence of two rapid tests – two days in a row – that are both negative is a very good indicator a person is no longer infectious,” Somers said. This doesn’t mean that being contagious at this point is totally impossible – it just means that once you have had Covid-19 for several days and then test negative with a rapid antigen test, the viral RNA is likely at levels low enough to not be infectious (even if you still test positive on a PCR test). There have been many online reports of employers requiring people to show proof of a negative PCR test before they return to work, which is “probably not coherent thinking” and has people not returning to work until weeks after their initial rapid antigen or PCR test, Ko said. “I would highly urge individuals to contact their HR (human resources department) – or if there’s a medical or Covid lead on their teams – and escalate the issue because it’s not reasonable,” Wen said. In addition to being more convenient, the most logical return-to-work testing strategy is using rapid antigen tests, Ko said. “Just as with any test for anything, if you use it for the wrong purpose, you’re going to get an inappropriate result,” Wen added. However, “if somebody has had multiple antigen tests that are negative, but they have symptoms of Covid, having a PCR test can help to confirm if they have it or not.” Some places, such as [Antarctica](https://www.cnn.com/travel/article/antarctica-travel-covid-19/index.html) and [Kosovo](https://xk.usembassy.gov/coronavirus/), require travelers to show proof of a negative PCR test before entering. “If the requirement is a PCR within like 72 hours of travel or similar, a lot can happen in those three days,” Somers said. “That negative PCR from a specimen that was taken three days before travel may not be any better than a rapid on the day of travel. “For the issue of somebody who had an infection and is recovering, I think this is where it’s really an administrative decision in terms of how much risk the country is willing to take (or) how much margin of error,” she added. “If a country is trying to be extremely strict in terms of keeping out all infections, then having a negative PCR will be a more sure way of doing that. However, it’s going to potentially prolong the amount of time between when somebody can enter the country compared to when they were infectious.” CNN’s Kaitlan Collins, John Bonifield, Nikki Carvajal, Eric Levenson and Arman Azad contributed to this story.
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