COVID-19 Tests, what are the differences?

Covid-19 tests fall into three main categories: PCR, antigen and antibody.

PCR tests – These tests detect disease by looking for traces of the virus’ genetic material on a sample most often collected via a nose or throat swab. The U.S. Centers for Disease Control and Prevention (CDC) considers PCR tests the “gold standard” of COVID-19 testing, but, like all tests, they are not perfect. Studies have suggested as many as 30% of COVID-19 PCR test results are inaccurate. (For comparison, the CDC in 2018 estimated that rapid flu tests have about the same rate of incorrect results.) This is done at a lab and requires around 48 hours to receive the results.

With COVID-19 tests, false negatives seem to be much more common than false positives—so if you get a positive result, you very likely do have the virus. If you get a negative result but have coronavirus symptoms or recently encountered someone sick with the virus, you should still self-isolate until symptoms subside.

Antigen tests – Antigen tests can turn around results in minutes—but speed comes with tradeoffs. Like PCR tests, antigen tests usually require a nose or throat swab. But unlike PCR tests, which look for genetic material from the SARS-CoV-2 virus, antigen tests look for proteins that live on the virus’ surface. This process is a little less labor-intensive than PCR testing, since there is not as much chemistry involved, but it is also less sensitive. This opens the door for possible false positives (if the test picks up on proteins that look similar to those from SARS-CoV-2) or negatives (if it misses proteins entirely). False positives are rare with antigen tests, but as many as half of negative results are reportedly inaccurate. If you test negative but are showing symptoms or have had a risky exposure, your doctor may order a PCR test to confirm the result.

Antibody tests – Unlike the other tests listed here, antibody tests are not meant to pick up on current infection with SARS-CoV-2. Rather, they search the blood for antibodies, proteins the body makes in response to an infection that may provide immunity against the same disease in the future. These tests look for SARS-CoV-2-specific antibodies to see if you have previously had coronavirus. Right now, antibody tests cannot do much except satisfy curiosity. For one thing false results are fairly common. Even if the results are accurate, scientists do not yet know how well or for how long coronavirus antibodies protect someone from a future case of COVID-19. A positive antibody test result does not mean you cannot get COVID-19 again, at least as far as current science suggests.

Wide-scale antibody testing is useful for researchers, since it could inform estimates about how many people have actually had COVID-19 and help scientists learn more about if or how antibodies bestow immunity to coronavirus.

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