As coronavirus antibody testing becomes increasingly available to the public, it’s worth understanding what these tests do and don’t reveal — and how to interpret their results.
Serology (antibody) testing to gauge Americans’ exposure to the coronavirus, heralded early on as a pivotal step to reopening the U.S. economy, has drawn increased attention in recent weeks.
The Centers for Disease Control and Prevention are gearing up to test up to 325,000 blood donors across 25 metropolitan areas for COVID-19 antibodies, according to a recent Reuters report. The research, starting in June or July, will help track the virus’s spread and the antibodies’ evolution over time, the report said.
Antibody testing is different from a molecular test, which is used to diagnose an active infection with COVID-19.
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The FDA issued stricter guidance for commercial antibody tests after it saw ‘unscrupulous actors.’
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Antibodies are proteins that form as a result of our bodies’ immune systems recognizing an invader, such as a virus, and then fighting it off, said William Schaffner, an infectious-disease specialist at Vanderbilt University. “You can consider them some of the soldiers that fight off the invader,” he said. These antibodies, which are specific to each virus, remain on site for some time into the future, depending on the virus, he said.
And if you have antibodies? “It’s like having soldiers,” Schaffner said, “to fend off that infection if it comes around to try to infect us again months, years later.”
Should you get tested?
In general, most people don’t need to worry at this point about getting tested for antibodies, said Bobbi Pritt, a clinical microbiologist at the Mayo Clinic. After all, the result shouldn’t have any bearing on the precautionary measures they’re taking.
Some people, including those who are immunocompromised, may find it useful to get tested to put their minds at ease, she added — “but they won’t want to change their behavior.” “They’ll still want to be careful, wear a mask, wash their hands and distance themselves from people in crowds,” she said.
Schaffner advised asking your health-care provider if you want to get a test. “If they’re available to you, and your health-care provider is convinced that this is now an accurate test that has been validated by the FDA, I don’t think there’s any reason for you not to, if you have that curiosity,” he said.
Daniel Leung, an infectious disease physician-researcher at University of Utah Health, recommended leaving it up to public-health and medical practitioners to initiate serological testing. “I know people are trying to take their health in their own hands,” he said, “but I don’t think a serology test is the right way to do it.”
American Medical Association president Patrice Harris issued a statement to that effect last week, warning that “physicians and the general public should not use antibody testing to consider anyone immune to the disease,” as “doing so may lead individuals to falsely assume they can stop physical distancing and further the spread of illness.”
“Although many are using these tests to determine whether an individual had COVID-19, we encourage physicians to only use antibody tests authorized by the Food and Drug Administration (FDA) and only for the purposes of population-level studies, evaluating recovered individuals for convalescent plasma donations, or along with other clinical information as part of a well-defined testing plan for groups or individuals,” Harris said.
The FDA issued stricter guidelines for commercial antibody tests
Researchers have tried to gauge what proportion of some regions’ populations might have been exposed to SARS-CoV-2, the virus that causes the COVID-19 disease. Preliminary results from serology testing last month in New York City, for instance, revealed that more than 21% of the sample had antibodies.
A similar effort in Boston this month found that nearly 10% of residents sampled from four zip codes tested positive, though the mayor acknowledged the results didn’t offer “a defined, city-wide picture”; a Los Angeles County study estimated antibody prevalence at 4.65%.
The Food and Drug Administration, which is using emergency-use authorization (EUA) to temporarily authorize select treatments and tests without issuing formal approval or clearance, had authorized 12 commercial antibody tests under EUAs as of Tuesday.
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‘What we don’t know yet is whether the antibodies evoked by COVID-19 actually indicate protection.’
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— William Schaffner, an infectious-disease specialist at Vanderbilt University
Earlier this month, the FDA issued stricter guidelines for commercial antibody tests after it saw “unscrupulous actors marketing fraudulent test kits,” tests that performed poorly under independent evaluation, and misleading promotion of commercial tests.
Antibody tests are available from labs and health-care providers, according to the CDC, as well as through some walk-in and drive-through testing sites around the country.
An antibody test is designed to test a specimen — typically serum, a component of blood — to detect whether someone has been exposed to the virus, Pritt said, but it’s possible to have false positive or false negative results.
Tests’ specificity (the ability to correctly identify those who have not been infected with the virus) and sensitivity (the ability to correctly identify those who have been infected with the virus) can vary, Leung said. The predictive value of a test depends on both the test’s performance and on the prevalence of people who have been infected in the population, he added.
What COVID-19 antibody testing can and can’t tell us
A positive test result for SARS-CoV-2 antibodies provides good evidence you’ve been exposed to the virus in the past, Pritt said. But the “million-dollar question,” she said, is whether antibodies for this particular coronavirus will keep a person from getting sick with COVID-19 again.
Antibodies will likely provide “some extent of protection,” she said. But scientists aren’t sure whether SARS-CoV-2 antibodies will fully protect against reinfection, nor whether they will protect against a severe infection the next time around, she said.
Schaffner echoed Pritt. “What we don’t know yet is whether the antibodies evoked by COVID-19 actually indicate protection,” he said. “We think they should, because that’s the way it is with most viruses — it just hasn’t been specifically demonstrated in this case yet.” It’s unclear right now how long that protection would last, he added.
“Some people will say, ‘Well, if I don’t know that for sure, how can I rely on this?’” Schaffner said. “Cut everyone some slack. Science takes time. This is a new virus.”
The best uses for coronavirus antibody testing
The strongest use case for serology testing right now is by public-health practitioners, said Leung. They use it to gauge what percentage of a population has been exposed to COVID-19, track outbreak trends, and see how well interventions like social distancing are working.
That information can then help guide policymaking and public-health decisions, Leung and his colleagues wrote in a recent journal article.
Aside from population-level surveillance, serological testing can also be used to identify patients who may be candidates to donate convalescent plasma, which could be used to help another patient who’s sick with COVID-19, Pritt said.
In some cases, Leung added, antibody tests can be used as a supplement to PCR tests (which tell a person if they currently have the virus) to confirm previous infection. “With the right symptoms and with clinical management decisions to be made, an antibody test can help your doctor with your care,” he said. Antibody testing could also be used in vaccine studies, he said.
When it comes to interpreting antibody test results, “you have to be very careful about giving people a false sense of security,” Pritt said.
“We really don’t know what a positive means,” she said. “What it’s not is a passport to go out and not wear a mask, and not wash your hands and not be careful, because we don’t know if that person is really, truly, fully protected.”
