PASCO COUNTY, Fla. -- They've been called an important part of the reopening process, but according to guidance on the Centers for Disease Control and Prevention's web site, antibody test results shouldn't be used to make decisions about returning to work or gathering people together in places like schools, dorms, and correctional facilities.

"I think the guidance from the CDC is building upon the reality that we still don't know a lot about this virus," said Dr. Jay Wolfson, a professor of public health, medicine, and pharmacy at the University of South Florida and senior associate dean of the Morsani College of Medicine.

The tests are meant to show whether people have antibodies that could protect them from COVID-19, but the CDC said in a test that has 90% sensitivity and 95% specificity, more than half of the results will be false positives. That's in a population where prevalence of the virus is five percent.

"When we have a test that's 95% specific, what that means is that five percent of people who are actually, truly negative, who've never been exposed to the disease, are going to test positive," said Dr. Kate Grabowski, an assistant professor in Johns Hopkins University's Dept. of Pathology.

Grabwoski said that's exacerbated in populations where prevalence of the virus is low, as it is in most places in the U.S.

Wolfson said it's hard to say how useful results are for individuals who've already been tested.

"For many viruses, once you get it, you can't get it again, that particular strain. But for this one, we're not sure about that," he said.

"Even if you have a positive test result, you still need to take the same precautions as everybody else," said Grabowski.

To improve the probability that people with positive test results truly have antibodies, the CDC's web site says tests can be chosen with specificities of 99.5% or higher, testing can be focused on people who had an illness with COVID-like symptoms, and those who test positive can be re-tested to lessen the likelihood of false positives.