Story at a glance
- The CDC has released interim guidelines about COVID-19 antibody or serology tests.
- If there is low prevalence of SARS-CoV-2 in the population, that could mean more positive results are false positives.
- Serology tests shouldn’t be used to determine what activities people can participate in, says the CDC.
Antibody tests may be giving more false positive results than expected depending on how common COVID-19 is in the population, also known as prevalence. The Centers for Disease Control and Prevention (CDC) warns that as many as half of positive results from antibody tests could be false positives depending on the sensitivity of the antibody test.
“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 <5% to 25%, so that testing at this point might result in relatively more false positive results and fewer false-negative results,” states the interim guidelines by the Centers for Disease Control and Prevention (CDC).
Low prevalence may then affect the probability that you’ll get a false positive in an antibody test for SARS-CoV-2. “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 says.
This may happen because of how all these numbers work together. Low prevalence in the population means the chances that the person being tested had SARS-CoV-2 is low. If you combine that with a test that has lower sensitivity and specificity, that lowers the chances that the test will properly identify a result. Put together, you may get more false positives. If the population has a high prevalence of COVID-19, there will be fewer false positives.
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The CDC says that someone with a positive antibody result cannot be assumed to have immunity to SARS-CoV-2 and potential future reinfection. “Everybody wants to believe that if I have antibodies, I’m immune,” says pathologist Alan Wu at University of California San Francisco (UCSF) on the university’s news site. “Well, we can’t be certain of that. The antibody test for this virus hasn’t been around long enough to show that nobody can get infected again if they have antibodies.”
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Antibody tests shouldn’t be used to determine what a person should and shouldn’t be able to do. “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,” states the CDC. Serologic test results should also not be used to make decisions about returning persons to the workplace.
Currently, antibody tests may not provide very valuable information to the individual. Serological data may be useful if it is being collected as part of a research study. Infectious disease specialist Chaz Langelier at UCSF says, “I think it’s important to know that early during infection, a negative antibody test does not mean someone is uninfected and that it takes time to develop an antibody response.”
For up-to-date information about COVID-19, check the websites of the Centers for Disease Control and Prevention and the World Health Organization. For updated global case counts, check this page maintained by Johns Hopkins University.
You can follow Chia-Yi Hou on Twitter.
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