The U.S. Food and Drug Administration (FDA) recently issued a letter warning healthcare providers about the limits of serological tests to detect SARS-CoV-2 antibodies. The agency is urging clinicians to "not use serological (antibody) tests as the sole basis to diagnose COVID-19 but instead as information about whether a person may have been exposed."

In mid-March, the FDA "provided regulatory flexibility" for test developers, the agency noted, which has resulted in scores of antibody tests quickly hitting the market without the agency's usual review. As of April 18, just four antibody assays had received emergency use authorisation from the FDA.

The agency said it "is not aware of an antibody test that has been validated for diagnosis of SARS-CoV-2 infection." The tests measure IgM or IgG antibodies, but IgM antibodies may not develop at all, and IgG antibodies usually do not develop until later in the disease process. Therefore, using such tests to diagnose COVID-19 will miss infections.