Testing around COVID-19 has been a mess. Tests have not been available, tests have not been reliable, which test do you get? It is all pretty confusing. Let’s lay out some ground work to help understand some of the testing for SARS-CoV-2, the virus that causes COVID-19.
When our bodies are exposed to a virus we first try to fight it off with our innate immune system. This includes substances such as our skin, mucous on our membranes, and non-specific types of white blood cells. These defenses are ready to go immediately whenever there is an invader. If the invader is not dealt with by our innate immune system, about 4-7 days later our adaptive immune system kicks in. Our cells start to make proteins that can attach to the invader, such as a virus, and tag them so that other cells can come and destroy the tagged invader. These proteins are called antibodies and are very specific to the invader they are trying to have killed.
When a person is thought to have the virus, whether they are acutely ill or asymptomatic, they need to have a test that will look for the actual virus itself. This is most commonly done via a nasopharyngeal swab that collects the RNA - the genomic blueprint of the virus. There is also a saliva test that has just been FDA approved. This small bit of RNA that is collected is then amplified by a method called polymerase chain reaction (PCR) so that there is enough material for a test to detect it. If this test is positive, the person has COVID-19. If this test is negative, the person could be negative. There also could be false negatives where the test is negative but the person really does have the disease. This could happen if there was a sampling error (did the swab get enough material?), a processing error (was the swab kept at the correct temperature?), or a timing error (too late in the disease process?). It is thought that the amount of virus tends to peak at or just before symptom onset followed by a rapid decrease. This means that the earlier this test is done, the more likely it is to be positive. Less than 7 days after the onset of symptoms is ideal, but can go out to 21 days after exposure.
When a person wants to know whether they have had the virus or not, regardless of whether they were symptomatic, they need to have an antibody test done. This is a blood test that checks if a person has antibodies to specific portions of the virus. This test is not used to diagnose someone with COVID-19. The antibodies start to rise about 5 days after an infection begins and lasts for likely months or more, but it is not known how long. The best time to test for antibodies is at least 15 days after exposure to the virus so the body has had time to mount an adequate antibody response. If a person has a positive antibody test it means that they have most likely had COVID-19. These antibody tests seem to be pretty specific, meaning if they are positive they are positive to the SARS-CoV-2 virus and not some other coronavirus they may have had in the past. The higher the prevalence of the disease in the community, the more likely the test is a true positive. There could be some false negatives with this test as well. The test could have been performed too early, a person may have an immune deficiency and not have an antibody response, or not have a robust enough antibody response to the particular viral protein to be detected by the test.
There are still lots of unknowns around this virus and testing. The biggest question is does having antibodies mean that a person is immune to the virus and won’t get reinfected? A recent study in monkeys shows that this is the case. It's yet to be determined in humans. And if it is true - how long does this immunity last? Only time and more scientific research will tell, but in the meantime we can support our immune system by having a nutrient dense diet, getting some exercise, a good night’s sleep, and managing our stress.