The CDC Issued This Grim Warning About Antibody Tests
The U.S. Center for Disease Control and Prevention revealed that current antibody tests for those who have been infected with COVID-19 are not reliable, casting a cloud over current testing progress so critical to safely reopening a U.S. economy cratered by the coronavirus pandemic.
In guidance posted over the weekend, the CDC noted that if the antibody test is used in a population where the prevalence of COVID-related sickness is low, it's possible that "less than half of those testing positive will truly have antibodies."
There are two sorts of tests that are critical for public health experts to identify the current risks and abatement progress of the COVID-19 contagion. Viral tests determine if one is positive for the coronavirus at the time of testing—and antibody tests—which determine if one's immune system has built antibodies to fight COVID-19. According to the CDC guidance:
In the current pandemic, maximizing specificity and thus positive predictive value in a serologic algorithm is preferred in most instances, since the overall prevalence of antibodies in most populations is likely low. For example, in a population where the prevalence is 5%, a test with 90% sensitivity and 95% specificity will yield a positive predictive value of 49%. In other words, less than half of those testing positive will truly have antibodies. Alternatively, the same test in a population with an antibody prevalence exceeding 52% will yield a positive predictive greater than 95%, meaning that less than one in 20 people testing positive will have a false positive test result.
While both the viral and antibody test results are critical metrics for medical and public health experts to consider, they measure very different things. In the case of antibody tests, the results determine if someone has recovered from COVID-19 or had it and was asymptomatic. It's akin to looking at a "rear-view mirror" of one's health. The antibody testing data is a crucial metric, though the incidence of false-positive results makes it relatively less useful according to the CDC.