Antibody tests for the new coronavirus are capable of being incorrect roughly half of the time and should not be used to determine whether physical distancing or use of personal protective equipment is no longer necessary, the Centers for Disease Control and Prevention said.

These serologic tests, which test blood for antibodies created in response to the virus, SARS-CoV-2, should be used for research and virus surveillance purposes and not to make public health recommendations or to determine immunity, the federal health agency said in new interim guidance posted to its website.

If only a small percentage of those being tested are infected, the tests’ margin of error will be higher, whereas if the rate of infection is higher among those being tested, the positive predictive value increases, the CDC explained.

“When a test is used in a population where prevalence is low, the positive predictive value drops because there are more false-positive results, since the pre-test probability is low,” the CDC’s website states.

“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 CDC said. It’s possible that in areas where only a small percentage of those being tested are infected, “less than half of those testing positive will truly have antibodies.”

Until more information is available, the CDC advises that serologic testing is used for research and virus surveillance purposes. Those with a confirmed past infection are also urged to continue to practice social distancing and other safety protections, especially when it comes to health care workers and first responders.

For now, the benefits of serologic testing include helping determine which communities may have experienced a higher infection rate and may have higher rates of herd immunity. It can help determine who can donate blood used to manufacture a possible treatment for those seriously ill from COVID-19. It can also help diagnose later complications of COVID-19 illness, such as multisystem inflammatory syndrome in children, the CDC said.

It was reported last week that the CDC began reexamining its national COVID-19 case count after separating the viral test results and antibody tests, partially because of how unreliable and confusing serological testing can be.

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