Antibody tests for Covid-19 wrong up to half the time, CDC says

Antibody tests used to determine if people have been infected in the past with Covid-19 might be wrong up to half the time, the US Centers for Disease Control and Prevention said in new guidance posted on its website, CNN reports.

Antibody tests, often called serologic tests, look for evidence of an immune response to infection. "Antibodies in some persons can be detected within the first week of illness onset," the CDC says.

They are not accurate enough to use to make important policy decisions, the CDC 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 CDC says.

"Serologic test results should not be used to make decisions about returning persons to the workplace."

Health officials or health care providers who are using antibody tests need to use the most accurate test they can find and might need to test people twice, the CDC said in the new guidance.

"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.