Some back-of-the-envelope math reveals the risk in relying on even the best antibody tests to tell us who’s had the coronavirus

 

Even with a test that correctly identifies antibodies in coronavirus-positive people more than 90% of the time, and no antibodies in coronavirus-negative people just as often, if you have a population where the actual prevalence of the disease is very low, the test can produce false results for half of those who take it, according to Dr. Andrew Noymer, a public health associate professor at the University of California Irvine. “Even when you log on to your healthcare portal, and it says, ‘Congratulations, you are positive for Covid antibodies,’ meaning you ostensibly have some immunity, it’s not like something you can take to the bank,” Noymer said in an interview.