COVID-19 Tests:
Everything You Need To Know

By now (February 2022), just about everyone has had at least one test for COVID-19. With school back in session, people traveling, and thousands getting sick, tests are in high demand. The question is, which test do you need and when? Let’s talk about the two types of tests that are currently available.

PCR Tests
PCR stands for polymerase chain reaction. This is a test that amplifies a piece of genetic material (RNA) from the virus. It only needs a small amount of genetic material to work so the tests are very sensitive. That means there are very few false negatives. It’s so sensitive that it can pick up infection in the very early days. It can also remain positive for a while after you’ve recovered from your infection.

Rapid Antigen Tests
Rapid antigen tests, or the tests you take at home, look for a protein made by the virus. These tests rely on a large viral load in your nose in order to detect the presence of the virus. Because of that, they are more likely to be positive while you are contagious. This doesn’t necessarily correlate with symptoms though. Some symptoms, like fever or body aches, are related to your immune response to the virus. Also, if you have enough virus to test positive on a rapid antigen test, you are contagious even if symptom free.

Both tests depend upon correct sample collection. It’s important to follow directions on a home test very closely. Make sure you are getting the cells from inside the nose. Mucous is not a good source for a COVID-19 test.

What to use When
PCR tests are best used early on and rapid antigen tests are best used to end isolation. If you have symptoms of COVID-19 (sore throat, congestion, cough, fever, diarrhea), a rapid test may be negative in the first 2 days – you should get a PCR test. If a PCR test is not available to you, stay in isolation and repeat a rapid antigen test for the next 2 days. If you’ve tested positive with a rapid antigen test, you do not need a PCR test to confirm that you do in fact have a COVID-19 infection. These tests are highly specific and unlikely to give a false positive.

After 5 days, a negative rapid antigen test tells you that you do not have enough viral load to be contagious. Thus, you can end isolation. A PCR test is NOT recommended to end isolation because as I mentioned, it can remain positive as it only requires a tiny amount of genetic material to be present.

This graph tweeted by Dr. Eric Topol from an article written by Michael Mina demonstrates when someone is infectious and when each test is best used. Lateral flow tests are another name for rapid antigen test.

Testing before activities
Going to a wedding? A concert? On vacation? Knowing you don’t have COVID-19 can be reassuring to you and those you will be with. The best time to use a rapid antigen test is right before you plan to gather with others. Viral load can change quickly so using a rapid antigen test days prior to your event is unhelpful. However, if you start to have symptoms later in the day, do another test.

PCR testing is sensitive enough to pick up the infection even before having symptoms, so these are helpful even 1-2 days before an event. However, as with rapid tests, it’s always good to retest if symptoms begin after your test was collected.

Overall, the COVID-19 tests, when used properly, provide important information to know what is causing your illness and how long you should be away from others. If you have any questions about your test or your illness, please contact your physician.