It seems like SARS-CoV-2 antibody tests should be pretty simple. Tests consist of SARS-CoV-2 antigens on a plate. If a person has antibodies for SARS-CoV-2, their antibodies bind to the antigens, and the test reads as positive. However, it is possible to get a false positive if some other antibody can bind by coincidence to the antigen.
“When the FDA originally came out with their recommendations, they were saying that you needed to have 95% specificity, which means if you tested 100 negatives, five of them could actually be false results,” said Abby Tyler, a scientist with Logan-based company Quansys Biosciences. Quansys Biosciences recently developed and deployed an antibody test for SARS-CoV-2.
A test that only returns false positives 5% of the time sounds pretty good on paper, but in areas like Rich County which have low incidences of COVID-19, a test with 95% specificity is dangerous.
“If you extrapolate that to a population of a thousand people that we're testing, and we know that only 10 people in that thousand actually have COVID. But your test is reporting five out of every hundred as false positives, then you're going to report that 50 or 60 have COVID when really, it's only 10,” Tyler said.
The take-home message is that, in areas of low incidences of COVID-19, it is crucial for antibody tests to return false positives only very rarely. Quansys’ antibody test, which is currently being cross-validated by the FDA, is estimated to return 3 false positives for every 1,000 tests performed.