Most of us aren’t biochemists or lab technicians. Some of us can’t even bake because it requires too much attention to precision measurements (read: me). So, of course, COVID testing is a complicated, elusive mystery to 99% of us. I’ll do my best to make it as clear as I can. For the purposes of this article, I’ll refer to “Sars-CoV-2” (the virus) and “COVID-19” (the disease) as just “COVID” (the lazy way).
There are three types of COVID tests available: PCR, antigen, and antibody.
Polymerase-chain reaction (PCR) COVID tests use a mucous sample from the nose or mouth to look for viral genetic material. If the test is run by a laboratory (as opposed to a rapid test done on the spot), PCR is considered the “gold standard” of COVID tests because it’s highly reliable; you can trust that if it says negative or positive, it’s correct most of the time (which is really the best you get in science.) These qualities make it a diagnostic test; it tells you a (mostly) definitive ‘yes, there is an infection’ or ‘no, there isn’t an infection’. Rapid PCR tests, which don’t use a lab, are less reliable, but pretty good.
Antigen COVID tests also use a mucous sample from the nose or mouth, but these tests look for proteins that are unique to a particular virus. It’s a rapid test, so it’s inherently less reliable than a lab-run test, but in exchange, you get results within minutes. The reliability of antigen tests depends on the case:
• If you have symptoms, it’s only reliable if the result is positive.
• If it’s negative, it needs to be confirmed by PCR or by repeated antigen tests every 3-7 days for 14 days from symptom onset.
• If you don’t have symptoms, either result is mostly reliable. Positive means you probably have an asymptomatic case of COVID. Negative means you probably don’t have COVID (right now), but if you need to know for sure, it needs to be confirmed, Which test is used for confirmation depends on your vaccination status and whether you’ve had a COVID infection in the last 3 months. These qualities make it more useful as a screening test; it’s good for testing asymptomatic people and can guide you on whether a symptomatic person needs PCR testing.
Antibody tests are blood tests that look for immune cells targeted to the virus. Antibody patterns are really specific to the pathogen of study (the bug you’re looking for). In the case of COVID, we don’t know the role of antibody tests yet. Some antibodies show up in hours, others in days, others in months. Some antibodies last for days, others for weeks, others for a lifetime. Some antibodies are only short-term and disappear once long-term antibodies develop. COVID antibody tests are not considered screening or diagnostic tests; they just provide clinical information.
A positive result means you have some immunity against COVID. Antibody tests can determine if your immunity is from a prior COVID infection or from vaccination only. But we don’t know how long COVID antibodies last or at what level we can consider you optimally protected, so results from antibody tests should absolutely not be used to guide you in your behavior.
Okay, so, what you really want to know is, what test do you need? That depends on whether you have symptoms and whether you need a negative test for something specific, like travel. If you do have symptoms, you can get a rapid antigen or rapid PCR test, but again, if it’s negative and you have symptoms, you should get the lab-run PCR to be sure.
If you don’t have symptoms but need a negative test for any reason, a rapid antigen or rapid PCR test is probably fine. But keep in mind that the party requiring the test probably wants a negative PCR because that’s the gold standard test and the most reliable. And if you don’t have symptoms but were exposed to a positive case, it depends.
If you’re fully vaccinated and/or have had COVID in the last 3 months, get tested 3-5 days after exposure. But if you’re not fully vaccinated and/or have not had COVID in the last months, you should get a rapid test immediately and every 3-7 days for 14 days from your exposure.
I know it’s confusing, but that’s where we come in. If you’re not sure what test to get or when, or if you even need to be tested at all, talk to your healthcare provider, and come up with a plan that fits your situation. Just remember you don’t have to figure it out alone.
Kayla D. Richardson, MD is Chief Medical Officer at River Valley Health & Dental. Let’s end COVID! invited her to help us better understand the different COVID tests.
Published 9/25/21
