CDC says antibody tests are wrong about half the time

Just weeks after antibody tests were touted as the ultimate basis for determining recovery time for the coronavirus, the CDC has updated its guidelines to indicate that the tests might be wrong up to half the time. 

Antibody testing, or serological testing, has been reviewed by both the CDC and FDA. New CDC guidelines indicate that tests which fall under this category may provide more false positives than false negatives.  

“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,” the new guidelines read.

In the most basic terms, this means that not enough people have tested positive for the coronavirus, which is necessary to be identified for an antibody test. The more people who are tested, the more accurate antibody tests can be. Currently, too few people have been tested for the margin of error to be reduced enough to come to a confident conclusion about whether enough people are testing positive for antibodies they actually have. It comes down to probability, where the sensitivity and specificity of an antibody test can produce a higher rate of false positives in lower prevalence rates. 

As CNN put it, “If just 5% of the population being tested has the virus, a test with more than 90% accuracy can still miss half the cases.”

With less than 1% of the U.S. having tested positive for coronavirus, as of the end of May, and no clear answer for the shortage of tests currently in production, antibody testing as a method of determining immunity has to take a backseat. 

Public health experts at the University of Minnesota advised that antibody tests shouldn’t be used to determine immunity just yet, because they believe that people who received false positive results would behave as though they are immune. 

Sources: CNN, CDC, CBS News

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