Antibody testing used to assess if people were contaminated with COVID-19 in the past could be incorrect up to half the time, the U.S. Centers for Disease Control and Prevention said in new guidelines posted on their website.
Antibody tests, also referred to as serological tests, seek evidence of an immune response to infection. "Antibodies in some persons can be detected within the first week of illness onset," the CDC says.
They are not sufficiently accurate to use for making important policy decisions, the CDC said.
"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 says.
The health agency clarified that serologic test results should not be used to make decisions about returning persons to the workplace.
Health officials or health care professionals who use antibody testing will use the most reliable test they can find, and they may need to test people twice, the CDC said in the new guidelines.
The health agency added that the prevalence of SARS-CoV-2 antibody is expected to be low, ranging from less than 5% to 25%, so testing at this point might result in relatively more false-positive results and fewer false-negative results.
It's an argument that public health experts have often made in recent weeks, but the CDC points out the issue in the latest antibody testing advice.
A false positive can lead someone to think that they were contaminated when they were not. There is little evidence now that an infection gives people immunity to later infection. Still, doctors are worried that people will behave as if they are immune if they get a positive test for the antibody.
The new CDC guidelines echo recommendations from the University of Minnesota and other organizations that warn against using antibody tests to make policy decisions.
The CDC explains why research can so often be incorrect. Much has to do with the way the virus is widespread in the population being studied.
"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%," the CDC said. Hence, less than half of those positive tests will have antibodies.
On the other hand, the same test in a population with an antibody prevalence exceeding 52% will yield a positive predictive greater than 95%. Hence, less than one in 20 people testing positive will have a false-positive test result.
Therefore, it is safer to use high-specific tests - which are unlikely to throw up a lot of false positives - because there are many cases in populations where doctors suspect, the CDC stated.
The Food and Drug Administration has also warned on the effectiveness of the antibody tests.