The US Centers for Disease Control and Prevention recently listed a compilation of new guidance on their website that stated antibody tests used to determine if an individual has been infected with Covid-19 may be wrong half the time. The tests specifically look for evidence of an immune response in the body to determine, and now the CDC is claiming that they shouldn’t be used to “make decisions about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities,” according to their website.
The CDC is now encouraging health officials not to use these tests to make decisions about when essential workers can return to their workplace if they’ve been infected. Health care providers should also ensure that when they do give patients an antibody test that they’re using the most accurate test possible and are able to test the individual twice to be sure that the result is accurate.
“In most of the country, including areas that have been heavily impacted, the prevalence of SARS-CoV-2 antibody is expected to be low, ranging from less than 5% to 25%, so that testing at this point might result in relatively more false positive results and fewer false-negative results. The higher the sensitivity, the fewer false negatives a test will give. The higher the specificity, the fewer false positives. Across populations, tests give more accurate results if the disease being tested for is common in the population. If an infection has only affected a small percentage of people being tested, even a very small margin of error in a test will be magnified,” the CDC said.
One of the biggest concerns is the false positives that some individuals are receiving. A false positive could lead someone to believe that they had the virus and have a certain level of immunity over it when they could’ve actually never had it at all. It’s also extremely important to note that even if the positive test result is accurate, no one is immune from this virus, and you shouldn’t believe that you have no chance at getting it again simply because you have the antibodies. We still don’t know how this virus fully works and mutates, so everyone must continue to ease on the side of extreme caution.
“It cannot be assumed that individuals with truly positive antibody test results are protected from future infection. Serologic [antibody] testing should not be used to determine immune status in individuals until the presence, durability, and duration of immunity is established,” (CDC).
In general, the CDC doesn’t want anyone to use antibody testing as a means of making certain policy decisions. We need to be looking at actual case numbers and rates of infection more than anything when it comes to loosening quarantining measures. They claim that this testing can be wrong so often because of how common the virus is now.
“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,” (CDC).
The best way to receive an accurate test result is to take a test with higher sensitivity. High sensitivity antibody tests are much less likely to give a false positive specifically. The Food and Drug Administration has joined the CDC in expressing their hesitation in using antibody results for policy change. For now, both organizations encourage double-testing, and remaining diligent on all other social distancing measures as well.