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The Food and Drug Administration is stiffening its rules to counteract what 

 a Wild West of antibody testing for the coronavirus.

These tests are designed to identify people who have been previously exposed to the virus. The FDA said more than 250 developers have been bringing products to the market in the past few weeks.

In a rush to make antibody tests available as quickly as possible, the FDA had set a low standard for these tests. Manufacturers were supposed to submit their own information about the accuracy of their wares, but the agency had no standards for what would be acceptable. Companies weren't allowed to claim the tests were authorized by the FDA, under initial guidance issued in mid-March.

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 that if they want their tests to remain on the market, they must meet minimum quality standards and submit a request for 

, a temporary route to market for unapproved products when others aren't available. The EUA involves a lower standard than the usual FDA clearance or approval.

The FDA said 12 manufacturers have already opted to request EUA's for their products. More than 100 other producers have been talking to the agency about using this process, said FDA Commissioner Stephen Hahn. He spoke on a press call Monday. Companies have 10 days to submit that request.

"Our expectation is that those who can't [meet the new standard] will withdraw their products from the market and we will be working with them to help them do that," he said.

These tests are now so widespread that people can order them from lab giants Quest or LabCorp. The tests can cost more than $100. Though the FDA's original guidance calls for these tests to be run by a certified lab, the kits themselves are simple to use and have been readily available.

Despite the enthusiasm surrounding these tests, they have substantial limitations. Though people who test positive for antibodies have in most cases been exposed to the coronavirus, scientists don't know whether that means those people are actually immune from the coronavirus, and if so for how long.

"Whether this is the ticket for someone to go back to work [based solely on an antibody test result], my opinion on that would be no," Hahn said.

The tests may be more useful when combined with information from a standard coronavirus diagnostic test, or in someone who has symptoms, or if the results have been confirmed with a different antibody test. That "would dramatically increase the accuracy of those tests," said Jeffrey Shuren, director of the FDA's Center for Devices and Radiological Health

Antibodies are a potentially valuable research tool, and can be used to determine the prevalence of a disease in a population. In that circumstance, individual false results are less important. New York State used antibody tests to determine that about 20 percent of people in New York City have already been exposed to the coronavirus.

In California, researchers have attempted to measure the prevalence of the coronavirus in Los Angeles County and Santa Clara County in the Bay Area. Those unpublished results have garnered criticism because even a test that's more than 99 percent accurate can produce many false positive results when used to survey hundreds or thousands of people. 

In the face of this criticism, the authors of the Santa Clara study 

 acknowledging the high degree of uncertainty in their findings. Those findings haven't been peer-reviewed.

The emergency use authorization is only valid during the time of the national emergency. "Once the national emergency ends, the EUA authorizations end as well," Shuren said. Companies that want to keep marketing these tests will need to get them approved through the regular, more stringent FDA process.

FDA officials say they will continue to crack down on companies that falsely claim their tests are approved by the FDA, or that market them for home use, which isn't currently allowed.

You can contact NPR Science Correspondent Richard Harris at 

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These tests are designed to identify people who have been previously exposed to the virus. The Food and Drug Administration is now telling manufacturers they'll have to meet minimum standards.

FDA cracks down on antibody tests for coronavirus

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