Coronavirus FAQ: My drugstore now offers antibody tests. Is it worth getting one?

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My drugstore has started advertising antibody tests. Does that mean I can tell how protected I am from COVID by forking over a fee — I've seen $10 to $140 — and a vial of blood?

Although antibody tests are increasingly available, the answer is unfortunately, no. We put this question to experts, who explained why these tests — also called serology tests — are not yet useful for most individuals, and why you definitely shouldn't change your behavior based on the results.

First, some background: A COVID antibody measures one component of your body's immune response to the virus or vaccine. Unlike PCR tests and often hard-to-find rapid antigen tests, an antibody test is not a test of whether or not you currently have COVID.

"It absolutely cannot tell you if you're [currently] infected," says Cynthia Leifer, a professor at Cornell University's Department of Microbiology and Immunology.

Antibody tests analyze a sample of your blood in order to determine whether your immune system has mounted one type of defense, either from being infected with the SARS-coV-2 virus or the vaccine. Specifically, the test tries to measure how many antibodies are in your blood. If your result is positive, it means you've likely had some form of exposure. But if it's negative, it could mean a few different things: That you haven't been exposed to the virus or the vaccine or that your antibodies have diminished to a point that the test can no longer detect them.

Early on in the pandemic, the idea of an immunity passport based on antibody levels seemed promising. But, "the thing that everyone wants to know — am I protected? — can't be answered with an antibody test," Leifer says.

There are several reasons why: First, the amount of antibody protection that people get from vaccines and exposures varies greatly — and not just by a small amount: If you sampled 100 people a month after they were vaccinated, their antibodies could differ by orders of magnitude, Leifer says.

And the amount of antibodies each person needs for protection likely varies as well, she says. Research, such as this study of 1,497 fully vaccinated health care workers in Israel, has shown a link between higher levels of antibodies and higher levels of protection. But no one can say exactly how many antibodies is enough.

Secondly, not all antibodies are the same, Leifer points out. Your body spits out IGM antibodies at the first sign of exposure, but then switches to IGGs, which are better at protecting you from viral diseases like COVID-19. You could have a lot of one type of antibody but fewer of the other, and that could affect your level of protection. But most tests won't tell you the difference — and even those that do are only marginally more useful, since there's no known standardized threshold that indicates how protected you are.

Finally, antibodies are – by a long shot – not the only thing that protects you from getting sick with COVID. T-cells that attack infected cells also come to your defense when activated by a vaccine or virus. And no current test measures that response. (Plus, there's a whole other arm of the immune system that also helps clear out the virus).

So a positive test will simply tell you if you've had the vaccine (which shouldn't be news to anyone!) or if you've been exposed but not how protected you are. If you haven't gotten the vaccine and you're tempted to take an antibody test to see if you might have been infected and not realized it, that still doesn't mean you'd have any guarantee against reinfection. And, regardless of the result, the recommendation from public health agencies is to get vaccinated.

Also, although the FDA has authorized dozens of antibody tests to identify individuals who "may have developed an adaptive immune response to SARS-CoV-2" — and the tests have become more affordable and available in other countries as well, including France and South Africa — the FDA does not recommend them as a means of testing your immunity level.

So is there any potential use for an antibody test?

Yes! In some immuno-compromised people, a doctor may advise an antibody test to get a general sense of how their bodies have responded to the vaccine, says Charlotte Baker, an assistant professor of epidemiology at Virginia Tech.

And these tests can be very helpful in studying the population as a whole, Baker and Leifer say. Antibody tests can help "researchers figure out how much of this [virus] is floating around," Baker explains. "And in research we use them to figure out how long antibodies last," she adds, which could help scientists determine the optimal timing and number of booster shots. Studies using antibody tests are going on now.

If you think an antibody test might be useful for personal health reasons, you can contact your doctor or a provider at an urgent care center or your health department, Baker says. Make sure to ask which test your clinic or drugstore offers, then check the FDA website for information on how accurate that specific test is and whether it will give you specific levels or simply a general "positive" or "negative" result.

But if you just really want to know if you've had COVID? Maybe someday, when more details are known about what exactly protects you, an antibody test will give you a meaningful result, Baker says. But for now, you're probably out of luck.

Sheila Mulrooney Eldred is a freelance health journalist in Minneapolis. She has written about COVID-19 for many publications, including The New York Times, Kaiser Health News, Medscape and The Washington Post. More at sheilaeldred.pressfolios.com. On Twitter: @milepostmedia. [Copyright 2022 NPR]