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 U.S. Centers for Disease Control and Prevention said in new guidance posted online, 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.
But these tests 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 ... Serologic test results should not be used to make decisions about returning persons to the workplace," the CDC said.
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.