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There are 2 major types of COVID-19 tests: diagnostic tests for active infection, and antibody tests for past infection.
Diagnostic tests look for components of the virus in a sample taken from the nasal cavity, throat, or saliva. Sample taken from the nasopharynx, the upper part of the throat behind the nose, is preferred when higher accuracy is required.
There are 2 types of diagnostic tests: molecular tests detecting viral RNA, and antigen tests detecting viral proteins.
Antigen tests use a technology similar to that of a pregnancy test. Some are made available as at-home test kits. The test is fast and less expensive, but is less sensitive. Antigen test gives positive results only with high viral loads, when the person is near the peak of infection, so it’s more likely to miss an active infection. In other words, the rate of false negative – a test that says you don’t have the virus when you actually do, is high. Symptomatic patients who test negative with rapid antigen test must be confirmed with a more sensitive molecular test. On the other hand, positive results are highly accurate, but false positive – a test that says you have the virus when you actually don’t, can still happen, most commonly due to errors in sample handling.
Molecular tests detect viral RNA. They are also called nucleic acid amplification tests, NAAT, because they amplify viral nucleic acids until there are detectable levels. Different tests are based on different technologies, with polymerase chain reaction, PCR, being just one of them. PCR is the gold standard for diagnostic testing but it requires specific equipment and takes longer to deliver results.
Molecular tests are much more sensitive than rapid antigen tests, but they can still produce false-negative results early in the infection. On the other hand, the high sensitivity may sometimes pick up the low viral load in a patient who has recovered and is no longer contagious. Positive results are highly accurate, most false positives are due to lab contamination or other errors with sample handling.
Antibody tests, also called serology tests, detect antibodies that the body produced in response to the infection. A blood sample is taken for this test. Because antibodies can take a couple of weeks to develop and may stay in the blood for weeks or months after recovery, a positive test result only proves that the person has been exposed to the virus. It gives no indication about active infection and should not be used to diagnose COVID-19.