What A COVID-19 Antibody Test Can (And Cannot) Tell You

What A COVID-19 Antibody Test Can (And Cannot) Tell You
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Antibody testing is designed to tell you whether you have been exposed to the coronavirus in the past, whether you had actual symptoms of COVID-19 or not. But it’s important to remember what an antibody test can and can’t do.

You might assume that if you test positive for COVID-19 antibodies, that suggest you have immunity and can safely relax physical distancing measures. But that’s not smart — health authorities says that we should all take the same precautions regardless of the results of antibody testing. The test could be wrong, and even if it’s right, we don’t yet know enough about the coronavirus to be able to say whether somebody with antibodies is immune to future infection.

For test results, context is key

When it comes to interpreting test results, context is everything. “There’s a concept in medicine and laboratory testing, which is called pretest probability,” says doctor S. Wesley Long of Houston Methodist Hospital. “What is the likelihood this patient has what you are testing for?”

In other words, if someone was sick with COVID-19 symptoms and tested positive for antibodies, then there’s a good chance that test result is accurate. If a person was hiding out on a secluded island, has had no contact with the outside world, and was never sick with COVID-19 symptoms but has still tested positive…well, there’s a higher chance that test result might be inaccurate. Given how new these tests are, it’s especially important to take context into account.

“If you’ve never had any symptoms, be careful how you interpret the results,” Long says. We still don’t know how common asymptomatic transmission is, so if you’ve never had symptoms but still got a positive result, that’s a situation that warrants additional caution — you certainly don’t want to assume immunity when none is present. And speaking of immunity…

Does a positive test result mean I am immune?

If you had COVID-19 symptoms in the past and got positive results from an antibody test, there’s a decent chance are you are immune. That said, there is still a lot we don’t know about COVID-19, and that includes how long any possible immunity might last.

“We don’t have enough data yet to know if these antibodies are truly protective against reinfection, and what that duration may be,” Long says. “We think that they are likely protective for many people, but we don’t know.”

It’s also very possible that a positive test could be wrong. Tests with high accuracy still have a small percentage of incorrect results, and false positives are especially common with these types of tests.

For example, the U.S. FDA calculates that a false positive on the test made by Assure is 80% likely to mean you really do have antibodies, if you live in a place where 5% of people have already had the virus. We explain the maths here, but the short explanation is simply that when true positives are rare, false positives are common. In most areas of the U.S., fewer than 10% of the population has had the virus.

Does a negative test mean I am not immune?

There is also the possibility of having a diagnosed case of COVID-19 and then still testing negative for antibodies against the virus. Some people, for whatever reason, don’t develop enough antibodies to trigger a positive result.

If you get a negative result, despite having had a confirmed case of COVID-19, this could be due to a couple reasons. One scenario is simply that, due to the sensitivity level of the test, antibodies against the virus are present but weren’t detected. This doesn’t necessarily mean you aren’t immune, it could just mean that your antibody levels are below the detection threshold of that particular test.

Another reason could be that, for some reason, you didn’t develop immunity despite getting sick. It’s very rare, but there are reports of children getting chicken pox multiple times, even though in most cases getting it once is enough to confer lasting immunity.

Check that your test is authorised by local health authorities

Before taking an antibody test, make sure to do your research on the specific test. “Not all antibody tests are created equal,” says Long. He emphasises it’s important to get a test that has been approved by a health body.

Early in the pandemic, this was especially important given that there were reports of really crappy tests flooding the market. This seems to be less of a concern now, but health authorities warned as recently as October that fraudulent tests, vaccines, and treatments are still out there.

This post was originally published in April 2020 and was updated on October 29, 2020 by Beth Skwarecki to include Kroger’s rollout of consumer level rapid testing, provide more information on the possibility of false positives, and remove early-pandemic concerns about organising society around who is and is not immune.

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