CDC: Antibody tests not to be used for decisions on returning to work

Antibody tests that determine if someone has had the coronavirus in the past should not be used for making decisions about people returning to work, the Centers for Disease Control and Prevention (CDC) says in new guidance.

The CDC raised concerns with the accuracy of the tests and said that even if someone has antibodies indicating they have already had the virus, it is unclear how long immunity from the virus lasts or how durable it is.

The guidance comes as a note of warning about antibody tests, given that some have expressed hope that antibody testing could help pave the way for certain people to return to work if they test positive and are shown to have already had the virus.

Antibody tests, also known as serologic tests, which determine whether someone has had the virus in the past, are different from diagnostic tests that determine whether someone currently has the virus.

“Serologic test results should not be used to make decisions about returning persons to the workplace,” the CDC says in the guidance on its website.

The tests also should not be used in making decisions about grouping people together at school or whether to return to school, the agency said.

“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,” the guidance states.

As an example of the accuracy problems with the tests, the CDC said that even if a test correctly identifies 90 percent of people who have had the disease as positive, and 95 percent of people who had not had the disease as negative, if the prevalence of the disease in the population is low, there will still be a large number of false positives.

“In other words, less than half of those testing positive will truly have antibodies,” the CDC said.

Even if someone really does have the antibodies showing they have had the virus already, it is not clear what level of immunity they have, the CDC said, noting research on that question is still ongoing.

The CDC said that while the presence of antibodies “likely indicates at least some degree of immunity, until the durability and duration of immunity is established, it cannot be assumed that individuals with truly positive antibody test results are protected from future infection.”

A wide range of antibody tests have been developed, and the Food and Drug Administration (FDA) sought to exert more control on assuring their accuracy earlier this month when it moved to require manufacturers to submit data to the FDA.

Former FDA Commissioner Scott Gottlieb said earlier this month that he would take an antibody test three times before trusting a positive result.

“If you do go out and get an antibody test and you get a positive result, meaning you have the antibodies, I would suggest you repeat it, because there's such a high false positive rate, meaning the tests say you have antibodies when you don’t, that I wouldn't put any stock in any single result,” Gottlieb said on CNBC.

“Quite frankly if it was me, I'd repeat it three times. I know they're expensive, but I wouldn’t put confidence in any one test,” he added.