CDC Says COVID-19 Antibody Tests Could Be Wrong Up to Half the Time

The CDC lays out the facts about antibody tests in new guidance shared to its website. The tests should not be used for getting people back to work.

May 27, 2020

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Antibody tests for COVID-19 could be inaccurate up to half the time, per the Centers for Disease Control and Prevention (CDC).

The agency, as spotted by CNN on Tuesday, makes note of this in a new guidance message posted to the Interim Guidelines for COVID-19 Antibody Testing section of its official site. The CDC—following similar cautionary assessments from various health officials—state that antibody tests, also referred to as serologic tests, should "not be used to make decisions" regarding the grouping of people in congregate settings, i.e. schools, dorms, etc. Furthermore, these tests should not be utilized as part of efforts to get people back in traditional workplaces.

"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," the CDC says. "For example, in a population where the prevalence is 5 percent, a test with 90 percent sensitivity and 95 percent specificity will yield a positive predictive value of 49 percent. 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 percent will yield a positive predictive greater than 95 percent, meaning that less than one in 20 people testing positive will have a false positive test result."

Also included in the CDC's guidance on antibody tests is a word of caution against individuals behaving with prematurely presumed immunity following a test, as well as three strategies the agency says could help improve positive predictive value. Previously, the CDC made it clear that it's not yet known whether having antibodies to the virus can actually protect a person from getting re-infected, nor is it known how long such protection could last.

There are two types of tests for COVID-19 available. The other is a viral test that can determine if a person is currently infected.