ST. PETERSBURG, Fla. — Antibody tests tell a person whether or not they have the COVID-19 antibodies in their system, indicating whether they've had the virus.
But the tests are not always perfect.
"One of the issues with a lot of the antibody tests that are out there, is how good they are," explained Dr. Michael Teng, a virologist with USF Health.
There are two different types of antibody tests: those that require a larger blood sample and are processed at a lab, and on-site, lateral flow device tests.
"A lot of the antibody tests that if you take a blood sample and send it off to a certified lab, those tests are pretty good," Teng said. The lateral flow tests, on average, are less precise and can be less accurate."
The state is using Cellex lateral flow tests at five state-run testing sites.
"Yesterday, the results were 16.1 percent were positive for antibodies. So that is a significant jump for where we were about a month and a half ago," said Gov. Ron DeSantis during a news conference Saturday. The state's report shows the Miami-Hard Rock site with that figure during the week of July 3-9.
Though, those tests are not always reliable. According to the Food and Drug Administration, the positive predictive value for the Cellex tests sits at 55.2 percent. What that means is if you get a positive result, saying you have antibodies, it is only accurate around half the time.
Even if you get a positive result, you might not have antibodies at all.
"So point-of-care tests are really good at picking up the people who are negative who don't have antibodies," said Teng, "but they're not as good as picking up people who are positive, who have antibody virus."
The Florida Department of Health released the following statement to 10 Tampa Bay regarding its use of Cellex tests:
"When the state purchased the Cellex tests, they were the only rapid antibody tests on the market that had received formal Emergency Use Authorization from the FDA. As you can see from the chart below, with information from the FDA, nearly every rapid antibody test available has a large range for positive predictive value, sometimes exceeding a 50 percentage point range. This is because positive predictive value is a calculation in part dependent on the percentage of people that have the condition being tested and not just a measure of the quality of the test."
The DOH also said it has seen the Cellex test perform well in the field with more than 40,000 results to date with positive antibody results "paralleling the changes in confirmed positive COVID cases in our communities."
It also said it is also using antibody tests from Healgen after 100,000 tests were received by the state this week.
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