Antibody, or serological, tests are designed to identify people who may have overcome covid-19, including those who had no symptoms, and developed an immune response. They are not designed to detect active infections. Some officials tout the blood tests as a way to reopen the economy by identifying individuals who have developed immunity and can safely return to work. But many scientists, as well as the World Health Organization, say evidence is lacking that even high-quality antibody tests can prove someone has immunity from the novel coronavirus and is not at risk of being reinfected.

The emergence of dozens of tests never reviewed by the FDA — many of which are being aggressively marketed — could confuse doctors, hospitals, employers and consumers clamoring for the products, according to critics who say the agency’s oversight of the tests has been lax. The questions are taking on special importance as federal and state officials debate strategies, including using serological testing, to help determine when they can end state and local lockdowns.

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“A test is only as good as its results,” said Kelly Wroblewski, director of infectious disease programs at the Association of Public Health Laboratories, which has been urging the FDA to take a closer look at the unapproved tests. “Having many inaccurate tests is worse than having no tests at all.”

Prodded by such concerns, the FDA recently stepped up warnings and is joining other agencies, including the National Cancer Institute, to try to determine whether the unvetted tests actually work.

On Thursday, during an interview with Washington Post Live, FDA Commissioner Stephen Hahn said “people should be very cautious” about tests that have not undergone the rigorous process of getting emergency use authorization from the agency. The FDA over the weekend said four tests have gotten such authorizations: Cellex, Chembio Diagnostic Systems, Ortho Clinical Diagnostics and Mount Sinai Laboratory.

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Much of the work validating the unauthorized tests will be done by the National Cancer Institute, working with the Centers for Disease Control and Prevention, the National Institute of Allergy and Infectious Diseases and academic laboratories. The cancer institute will use its human papillomavirus (HPV) serology testing lab at Frederick National Laboratory for the work. The assessments also will be conducted for some tests seeking FDA emergency use authorization, said Hahn in a statement Saturday evening. The goal is to have “greater confidence in test performance,” he said.

Meanwhile, the unreviewed tests remain on the market. In mid-March, the FDA, wanting to ensure quick access to serological tests, said manufacturers could sell them after validating results themselves and simply notifying the agency. The labels on the products must state they aren’t FDA-approved or meant to be used as the sole basis for diagnosing active infections. The agency lists on its website the companies and laboratories that have submitted those notifications. In response to concerns the list might be seen as an endorsement, the agency updated its website to make clear which tests are authorized by the FDA and which aren’t, Hahn said.

Testing experts warn the risks of inaccurate tests are high. A wrong result could, for example, indicate individuals have immunity against the virus when they don’t, potentially resulting in behavior that would endanger themselves and their families. The peril of buying unvetted tests was underscored by the British government’s recent $20 million purchase of antibody tests from China that didn’t work. Even for high-quality tests, scientists say, they don’t know the level of antibodies needed to make someone immune from the virus or how long protection might last.

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Many of the unreviewed tests being sold in the United States are made in China or elsewhere in Asia. Distributors staunchly defend their products as a way to accelerate testing in a country that has been hobbled by shortages and to provide individuals, employers and governments with critical information.

Jonathan Cohen, president and chief executive of 20/20 GeneSystems, based in Rockville, Md., said the test he is selling to medical professionals and clinical laboratories, named “CoronaCheck,” has been approved by the Chinese equivalent of the FDA and verified by hospitals in the United States. “These tests are quite good,” he said.

Paul Kahlert, owner and president of ARCpoint Labs of Edina, Minn., said customers are eager to know whether they were infected and also whether they may be able to donate blood that can be used to manufacture “convalescent plasma,” which is being tried as a potential treatment for people with covid-19. “People are so appreciative we are doing this,” he said. “They want to help.”

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But an information sheet about the ARCpoint antibody tests raises concerns, said Scott Becker, executive director of the Association of Public Health Laboratories. The sheet said the presence of certain antibodies suggests a person has “functional immunity” and can “discontinue social distancing.” Becker said scientists don’t know enough to say that about antibody tests for the coronavirus.

Amanda Harley, a spokeswoman for ARCpoint Labs, a franchise based in Greenville, S.C., said the information sheet was created as a guide for franchisees and is no longer used. John Constantine, the company’s president and chief executive, said the firm also provides disclaimers about its product, such as that test results are not a guarantee of immunity. “We would never say, ‘You are good. Go see grandma without a mask,’" he said.

The agency’s willingness to allow tests on the market without review means “you are getting a lot of folks where this isn’t their core competence entering the market, and there is an opportunity for flawed results in that environment,” said David Morgan, head of transplant and infectious disease at Eurofins U.S. A subsidiary, Boston Heart Diagnostics, is offering an antibody test to hospitals and is seeking FDA authorization, company officials said.

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Antibody tests typically come in either rapid versions that use a small amount of blood from a finger prick and may cost as little as $20 or less, or others that rely on traditional blood draws and cost considerably more. Many of the finger-prick ones are being marketed for use in doctors’ offices and pharmacies, even though the government requires all unauthorized tests be run in CLIA laboratories certified to conduct high complexity testing, Becker said.

That requirement is slowing use of the antibody tests, say suppliers who argue doctors should be able to run point-of-care tests in their offices without using sophisticated labs.

Some state and local health departments also have warned consumers and doctors to avoid tests that haven’t been authorized. In San Diego County, officials shut down a pop-up antibody testing site at a community college, citing a lack of documentation about the test.

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Some unreviewed tests have been developed by high-quality academic medical centers and aren’t a source of concern among public health experts. Emory University School of Medicine, for example, created a test it is planning to use for staff and patients to see how broadly the virus has been transmitted in its community.

Marybeth Sexton, an Emory infectious disease specialist, got the test because she wondered if a bout of illness in February was caused by the coronavirus. The test detected antibodies to the virus, but because she is still uncertain whether she’s immune, she plans to continue social distancing and wearing a mask. “I won’t do anything different,” she said.

In New York City, periodontist Scott Froum said he is exploring offering antibody tests to staff and patients when he reopens for routine care as an extra layer of protection against the virus. But he’s wrestling with regulations and choices. “You have to be careful,” he said. “Some of the tests don’t look kosher.”

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The FDA is expected to authorize more tests in coming days and weeks. To get the volume of serology testing the nation needs, officials say, more big manufacturers and laboratories will have to get involved. One big manufacturer, Abbott, launched a serology test and plans to produce 20 million tests a month by June. The company said it will apply to FDA for official authorization.

Carolyn Y. Johnson contributed to this report.