According to a report posted on the US Centers for Disease Control and Prevention website, antibody tests used to find out if people have been infected with COVID-19 in the past might be wrong up to half the time according to CNN.
The antibody tests, usually called serologic tests, look for proof of an immune response to infection. “Antibodies in some persons can be detected within the first week of illness onset,” the CDC says.
“Serologic test results should not be used to make decisions about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities,” the CDC states. They also say that “Serologic test results should not be used to make decisions about returning persons to the workplace.”
The new CDC guidelines caution against using antibody tests to make policy decisions. They explain why testing can be wrong so often. It has a lot to do with how common the virus is in the population being tested.
“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,” the CDC said.
“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.”
The CDC is stressing that everyone should still continue to practice preventative measures, including social distancing, proper hygiene, and wearing personal protective equipment, regardless of whether people have tested positive for antibodies or have recently had the coronavirus.
“It cannot be assumed that individuals with truly positive antibody test results are protected from future infection,” the CDC said. “Serologic testing should not be used to determine immune status in individuals until the presence, durability, and duration of immunity is established.”