The fact that COVID-19 can spread in the absence of symptoms makes testing for the virus essential for controlling transmission. Selecting an appropriate test involves two choices, what to sample and how to test it.
Tests for the presence of the virus rely primarily on samples taken from the back of the throat. These are taken using a long cotton swab, usually inserted though the nose. These are seen as the most reliable samples but can cause gagging and discomfort. Newer methods use either saliva or a sample take just inside the nose. These samples are less uncomfortable. Hopefully, they will soon be good enough for general use.
There are two general categories of tests, PCR tests and antigen tests. PCR is a method for duplicating genes from the virus over and over. This ability to make copies means it can detect minute quantities of the virus. However, it requires expensive laboratory equipment and can take hours to perform. Antigen tests detect the presence of proteins from the virus. They require little or no equipment and only a few minutes to run. They are not as sensitive as the PCR tests because they don’t involve a duplication step. Their ease of use and low cost allows them to be done more frequently. Also, it appears that the cases they miss may have levels of the virus that are too low to transmit the disease.
One other category of test is antibody tests. These tests use blood samples to look for antibodies to the virus. The presence of antibodies indicates past infection. It may also mean that a person is immune to further infection, but this remains to be verified.