A sign outside Principle Health Systems and SynerGene Laboratory lists COVID-19 tests available Tuesday, April 28, 2020, in Houston. The company, which opened two new testing sites Tuesday, is now offering a new COVID-19 antibody test developed by Abbott Laboratories. Credit: David J. Phillip | AP

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Maine is warning health care providers to use caution when interpreting the results of COVID-19 antibody test results, which are now available through some commercial laboratories. The Maine Center for Disease Control on Friday issued an alert stressing the antibody test’s limitations as they become more widely available.

There are two main types of tests for COVID-19: viral tests and antibody tests. The more common viral test requires a health care provider to swab patients’ nose or throat. It looks for evidence of an active viral infection.

[Our COVID-19 tracker contains the most recent information on Maine cases by county]

The antibody test, which has not been formally approved by the U.S. Food and Drug Administration, is a blood test that looks for evidence that people had the virus in the past, even if they didn’t have symptoms. It cannot diagnose people with COVID-19, and it’s not clear how accurate the tests are at detecting COVID-19.

“The big problem with antibody testing right now is that it’s not clear how specific the test is for COVID-19. The test definitely will detect coronavirus, but there are seven coronaviruses that affect humans. Four of them cause the common cold, which most everybody gets,” said Nirav Shah, the director of the Maine CDC.

“So if you test someone with an antibody test, and it’s not specific for COVID-19, and it comes back positive, we don’t know if the person had COVID-19 or if they just had a cold six months ago,” he said.

Antibody tests should not be used on patients who are symptomatic, the Maine CDC reminded providers. That’s because antibodies frequently do not appear until one to three weeks after infection. Some people may take even longer to develop antibodies, and some people may never develop them.

Therefore, there may be false-negative results from using antibody tests too early following symptoms and false-positive results if the tests detect other common coronaviruses.

People are being encouraged to continue to follow social distancing guidelines regardless of their results. That’s because if someone’s antibody test is negative, it likely means the person did not have a previous infection. But it does not rule out the possibility of a current infection.

It’s also unclear if antibodies can provide immunity against getting infected again.

“It is not known how a positive test, or specific antibody levels, correlate with a person’s immunity,” the Maine CDC said. “Nor is it known how long protection may last.”

Several clinics have started offering the antibody tests in Maine. People can now also purchase COVID-19 antibody tests for themselves online, without needing to visit a doctor’s office, through Quest Diagnostics.

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Erin Rhoda

Erin Rhoda

Erin Rhoda is editor of Maine Focus, a journalism and community engagement initiative by the Bangor Daily News.