When to take an antibody test for the coronavirus to get the most accurate results

• Antibody tests can help people figure out if they've had the novel coronavirus in the past.
• Your body creates antibodies to fight off infections like the coronavirus. Those antibodies stick around in the blood, after the virus is gone.
• It's still not clear whether people who've had the coronavirus are immune to it, or how long that immunity might last.
• The research on antibody testing is consistent on at least one front: you should wait at least two weeks after the onset of coronavirus symptoms before taking one.
• In several recent studies, the tests' ability to find antibodies among those infected, a metric called "sensitivity," increased with time across dozens of brands.
• Visit Business Insider's homepage for more stories.

While there's a lot we still don't know about the accuracy of antibody tests and what they can tell us, the research is consistent on at least one front: you should wait at least two weeks after the onset of coronavirus symptoms before taking one.

Antibody tests are the only way providers can figure out if someone had the novel coronavirus in the past.

Tests for the virus itself are used to tell whether someone is currently infected, but they can't find the germs once patients recover. Compare that to antibodies, which stick around for some time in people's blood.

In several recent studies, the tests' ability to correctly identify antibodies among those infected, a metric called "sensitivity," increased with time across dozens of brands. Many of their scores jumped from less than 50% to more than 80% just by using samples that were 10 days older.

Read more: Tests that can tell who's had the coronavirus are crucial to reopening the country. Here are the companies racing to bring them to the US healthcare system.

"These antibody tests are really detecting our immune response to the infection. That takes time to develop, especially since we've never seen this virus before," said Elitza Theel, the laboratory director of infectious disease serology at the Mayo Clinic, in an interview with Business Insider.

You could be tested too early

Theel's lab is testing several commercial manufacturers' antibody kits for use in studies at Mayo, according to Theel. One test followed 50 mostly hospitalized patients with confirmed cases of COVID-19, the disease caused by the novel coronavirus, and found that more than 95 percent of them had IgG antibodies after 14 days of symptom onset, Theel said.

IgG antibodies typically provide some level of protection against getting infected again by the same virus, she said.

That said, antibody tests shouldn't necessarily change people's behavior, since individuals can have antibodies and also still have an active coronavirus infection, according to Theel.

Read more A study backed by Mark Zuckerberg reveals companies are selling antibody tests for the coronavirus with big potential for error.

Forward, a preventive primary care practice, has seen similar results from internal data on more than a thousand patients in San Francisco, New York, Washington D.C., and more, according to Nate Favini, the medical lead.

"If you test in that window between when symptoms start and 14 days, your body is just going to be starting to make those antibodies," he said in an interview. "There could be a case where you test too early."

When people have coronavirus symptoms, Forward physicians are recommending tests that look for the virus itself, not antibody tests, he said.

Similarly, Quest Diagnostics says people with symptoms of the coronavirus shouldn't come in for its antibody test.

Whether COVID-19 antibodies confer immunity and for how long is the subject of ongoing research. For some diseases, antibodies can protect people for years after the initial infection. But for common coronaviruses, it's more typical to see reinfection after a shorter period of time, Mayo's Theel said.

It's better to wait to get an antibody test

One study looked at samples from sick people confirmed to have the coronavirus.

When they were tested for antibodies less than 14 days from when people started showing coronavirus symptoms, the tests found antibodies 40% to 86% of the time. When samples were tested 21 days or more days after symptoms started, the same tests found antibodies 78% to 89% of the time.

Another study, backed by the Chan Zuckerberg Biohub, a research center founded by Facebook CEO Mark Zuckerberg and Dr. Priscilla Chan, indicated that a dozen antibody tests also improved with time. They did the best when samples came from people who'd had the virus at least 20 days earlier.

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