PCR vs Rapid Antigen Test

Quick Guide To COVID-19 Testing

There are different types of COVID tests – diagnostic tests and antibody tests. Most people will take either a diagnostic PCR test or a rapid antigen test. Currently, antibody tests are not widely used for COVID-19. Each test helps in detecting COVID but in different ways. See the guidance below for the differences and when it is recommended to use a PCR test and a rapid antigen (at-home) test. Though the PCR test is considered to be the most accurate, there is a time and a place for an antigen test as well.

FAQ 

What is a PCR test?

  • A PCR test is a diagnostic test that can show if you have an active COVID-19 infection and need to take steps to quarantine or isolate yourself from others.

  • PCR -- or reverse transcription-polymerase chain reaction -- tests can detect small amounts of the coronavirus genetic material in a specimen collected, typically through nasal or throat swabbing. The test then works by amplifying, or making copies of, that genetic material if any is present in a person's sample.

  • Tests are processed in a lab and typically take 24-48 hours for results.

  • PCR tests are highly sensitive and accurate tests and are the gold standard for diagnosing SARS–CoV–2.

When are PCR tests a good option?

  • PCR tests should be used if you have symptoms or have been identified as a close contact with someone who tested positive, to identify if you are infected.

  • PCR tests can detect a much smaller amount of the virus and can therefore determine whether you’ve contracted the virus much sooner than an antigen test.

  • PCR tests are good for population-level surveillance, like on our campus, since they are more accurate (especially in people without symptoms), however longer turnaround times means they are less helpful in quickly identifying people who are more likely to be infectious.

What is an antigen test?

  • Antigen tests detect viral proteins, and are most likely to be positive while you are infectious.

  • Home antigen test kits are available over-the-counter at most pharmacies, or can be ordered online.  They are also performed at some medical clinics. There are MANY different brands, with different reliability, however most detect the Omicron variant.

  • To run an antigen test, you first treat a sample with a liquid containing salt and soap that breaks apart cells and other particles. Then you apply this liquid to a test strip that has antibodies specific to SARS-CoV-2 painted on it in a thin line.  Just like antibodies in your body, the ones on the test strip will bind to any antigen in the sample. If the antibodies bind to coronavirus antigens, a colored line appears on the test strip indicating the presence of SARS-CoV-2.

  • Faster, easier to perform, cheaper (doesn’t require a lab to process), no special training needed to perform and interpret results, the test does that for you and produces results typically in 15 minutes.

When is an antigen test of good option?

  • These tests are most accurate when there are more virus particles in your system — when people are likely to pass the virus to another more quickly.

  • Rapid antigen tests are sometimes used before gathering with vaccinated family and friends or before travel to provide some  reassurance, however keep in mind that a negative result in asymptomatic people is only 60-80% predictive of infection.

  • If a rapid antigen test is more available or convenient, it’s a good idea to use it. If it’s positive, you can generally rely on the result.  Negative results are less reassuring.

  • If you begin to exhibit symptoms, and the antigen test comes back negative, the best course of action is to confirm with a PCR test, or to repeat the antigen test 24 hours later.  With Omicron, it appears vaccinated people may often have a false negative antigen result the first day of symptoms.

  • A rapid antigen test is good to take after you have had COVID for five days, and want to check to see if you are still highly infectious to other people.

What type of test to use to find out if you are postive?

  • PCR Tests are most accurate, if you can get one.

    • If the test results are positive, you are positive, and you should not re-take a PCR test. Your PCR test may be intermittently positive for 90 days following infection and neither a positive or negative follow-up result will change management.

    • If the results are negative, you do not need to retake a PCR test, unless you develop new symptoms, it has been less than 5 days since your last exposure, or you have a new exposure. 

  • Rapid Antigen Tests are less accurate overall, but are faster, and are very reliable (likely >90%) if you have symptoms and are on day 2 - 5 (even more so if you have two negative antigen results 24 hours apart).

What type of test to use once you are positive and want to see if you can stop isolating at home after 5 days?

  • Antigen tests are most useful for early release from isolation.  

    • If negative, and you otherwise qualify (see GRID), you can go back to work/school, but you must wear a mask for the full 10 days following your initial positive result. 

    • If your antigen is still positive at day 5 (30% chance), you should continue isolating until it has been a full 10 days since your initial positive test (may be longer if immunocompromised, consult your physician).  

  • Once you have a positive test (either PCR or antigen), you should not take a PCR test for 90 days. It may still be positive and does not correlate well with infectiousness.

 What is the difference between an antigen and PCR test?

  • When a person is shedding a lot of virus, antigen tests are very accurate. However, unlike PCR tests, antigen tests don’t amplify the thing they are looking for. This means there needs to be enough viral antigen in the sample for the antibodies on the test strip to generate a signal. When a person is in the early stages of infection, not a lot of virus is in the nose and throat, from which the samples are taken. So, antigen tests can miss early cases of COVID-19. It’s also during this stage that a person has no symptoms, so they are more likely to be unaware they’re infected.

Which is better: PCR or antigen?

  • The answer is nuanced. Both tests are helpful in particular situations. The PCR COVID test is the gold standard. It is excellent in diagnosing COVID when you are developing symptoms. Simply put, a positive test, whether PCR or antigen, should be trusted as a positive test. A negative PCR test should be trusted as negative. A negative antigen test may need to be repeated within 24 hours or confirmed with a PCR test, if symptoms develop.

Do antibodies mean you are immune to COVID or from getting it again?

  • We don’t really know what the presence of COVID-19 antibodies means. It doesn’t necessarily mean that you’re immune to the virus. Medical science has yet to determine what level of antibodies confirm immunity, if immunity is possible, or how long immunity might last. The result of a positive antibody test indicates past infection and/or immunization, but may not correlate with immunity.