More and more health care providers in Maine have begun to offer antibody testing for the coronavirus, but they also are cautioning patients about the test’s limitations.

Antibody blood tests, also called serological tests, cannot diagnose a patient with COVID-19, but can tell whether someone has formed antibodies from being exposed to the virus. Antibodies develop in the body in response to infections and then help guard against future exposure to the same pathogen.

Testing for antibodies can help determine how widespread the virus is – especially because of the ongoing shortage of the more common polymerase chain reaction, or PCR, tests that are done with nasal swabs and determine if someone has the disease at the time of the test.

But experts have raised alarms about the accuracy of antibody tests, none of which has gone through the traditional review process conducted by the U.S. Food and Drug Administration. They also worry about giving patients a false sense of security. In most cases, antibodies create at least some immunity, but that has not yet been proven for COVID-19.

Dr. Andre Couture, medical director of AFC Urgent Care in South Portland, said his clinic began offering antibody tests this week, working with Quest Diagnostics, a leading testing services company, and using tests developed by Abbott Laboratories, which has a facility in Scarborough.

Couture said he and his staff are explicit with patients about what the test does and doesn’t do.

“The presence of antibodies does not make you immune. People who test positive still should follow CDC guidelines on physical distancing,” he said. “It does not mean you can go see your grandkids in safety.”

As of Thursday, AFC had conducted about 100 tests, three of which came back positive.

Other health care providers, including Intermed, a physician-owned medical group in southern Maine, are offering antibody testing but it’s not clear how widespread the practice is yet. Maine Center for Disease Control and Prevention spokesman Robert Long said entities that offer antibody tests are not required to notify the state before offering them.

“However, all positive tests for COVID-19 must be reported so our epidemiological staff can immediately begin contact tracing,” he said in an email. “Maine CDC is working with labs as they bring on antibody testing to ensure that they report results in a timely and accurate manner.”

Antibody tests have been rapidly developed across the country without FDA review. According to federal data this week, at least 150 antibody tests are on the U.S. market without having been reviewed. Another 24 labs are testing without FDA authorization.

So far, no antibody test for coronavirus has been FDA approved, although some have received what’s called an emergency use authorization, including Abbott’s.

One of the major problems with antibody testing at this stage is that they haven’t proven accurate.

A recent study by the COVID-19 Testing Project, a consortium of researchers and physicians at the University of California-San Francisco, the University of California-Berkeley, the Chan Zuckerberg Biohub, and the Innovative Genomics Institute, found some had high rates of false positives.

Of the 12 antibody tests that were studied, four had false positive rates of at least 10 percent and one had a rate above 15 percent. Dr. Caryn Bern, one of the study’s authors, called that “really terrible,” in an interview with CNN.

Even the standard swab tests used to diagnose COVID-19 are not 100 percent accurate, but experts say a false positive rate of 5 percent or lower is a good benchmark and 2 percent or lower is ideal.

A false positive on an antibody test could lead someone to think they are immune to the virus at a time when scientists still don’t know if people can develop immunity to COVID-19. Nevertheless, the Trump administration has floated the idea of issuing “immunity certificates” to those who’ve already had the virus, giving them less-restricted access to society.

Additionally, the U.S. Center for Medicare and Medicaid Services (CMS) has said antibody tests, once broadly available, “could become a key element in fighting the pandemic,” and has required insurers to cover the costs of such tests.

Maine Department of Health and Human Services Commissioner Jeanne Lambrew was asked this week about antibody tests and said Maine is exploring all its options for adding antibody tests. But she also said the accuracy and reliability of such tests are not yet where they need to be.

Maine Center for Disease Control and Prevention director Dr. Nirav Shah also has cautioned about the accuracy of antibody tests, but hopes they are part of the equation going forward.

“Right now in the scientific community, there are not great answers in knowing what the value of anti-body testing is,” Shah said this week.

One potential benefit of the test is that if people do develop immunity, they can donate antibody-rich plasma to those with the virus, which could boost immunity.

And if experts learn that people do develop immunity, the data being collected will help determine who might donate if and when cases spike.

“Our main goal is for patients to have more information about their overall health and to educate them,” said Couture, at the South Portland urgent care clinic.