So You Got Your Antibody Test Results Back. Now What?
What to make of a positive, negative, or equivocal result
The accuracy of antibody tests is still highly debatable, but they’re quickly becoming available in the U.S. These tests look for Covid-19 antibodies in the blood, which the immune system makes in response to an infection. The idea is that if you have these antibodies, you previously had Covid-19 — something you might have wondered if you started coughing or felt even a little warm in the past few months.
If you get an antibody test, your doctor or clinic will tell you the results in a couple of days. There are three likely possible outcomes: positive, negative, or equivocal. Seems simple, but interpreting these results can be complicated.
It’s important to note up front that the accuracy of the many different antibody tests on the market varies a lot, and even those that claim upward of 99% accuracy, like the one produced by Abbott, can’t tell you with absolute certainty that you did or didn’t have the disease. Knowing that depends on the prevalence of Covid-19 in your area, which also varies widely depending on where you are: For example, a 99% accurate test in an area with 1% prevalence only gives you results with 50% certainty (to understand the math, check out these really smart explainers in Scientific American and Vox), so take all results with a grain (or many) of salt.
But if we assume for a moment that the results of your test are 100% accurate, here’s what to make of them.
If you test positive for Covid-19 antibodies, it means you were previously infected with Covid-19. What it doesn’t mean is that you are now immune to Covid-19. Scientists still don’t know whether the mere presence of antibodies means you are immune. This may seem confusing since infection to diseases like chickenpox or measles confers lifelong immunity, but the body, unfortunately, doesn’t remember all viruses equally well.
Scientists also don’t know whether you need a certain titer, or amount, of antibodies in your system, to fight off an infection if you encounter the virus again. Most antibody tests can only give you a yes or no answer, and even if you did get a test that told you how many…