Why Antibody Tests Won’t Help You Much

Getting an antibody test to see if you had Covid-19 months ago is pointless, according to guidelines issued this week by a major medical society.

Many tests are inaccurate, some look for the wrong antibodies and even the right antibodies fade away, said experts at the Infectious Diseases Society of America, which issued the new guidelines.

Because current tests cannot determine if someone is immune, the society said, they “cannot inform decisions to discontinue physical distancing or lessen the use of personal protective equipment.”

Antibody testing generally should be used only for population surveys, not for diagnosing illness in individuals, the panel said. But its guidelines described two situations in which antibody testing could be used when the normal diagnostic tests for the virus — called PCR tests — failed or were likely to fail.

People who felt sick weeks or months ago and now wonder if they are immune to Covid-19 “probably shouldn’t bother” getting an antibody test, said Dr. Angela M. Caliendo, a testing expert at Brown University’s Alpert Medical School and a member of the society’s expert panel.

Many New Yorkers fell ill in the spring, could not get tested then and now wonder whether they had the disease and are immune; the panel concluded that current antibody tests cannot answer that question.

“We don’t really know if a positive test means you’re protected,” Dr. Caliendo said. “If you were sick back in March, you probably don’t have antibodies any more; and if you were only mildly sick, you may not have gotten antibodies.”   ...