Why Antibody Tests Won’t Help You Much

Ideally, it should be given not immediately after symptoms appear, but three to four weeks later — “the sweet spot,” Dr. Caliendo said.

(PCR nasal swab tests may come up negative after the virus migrates from the nose and throat to the lungs, Dr. Caliendo explained; but the body may not produce enough antibodies to be detectable in the blood until more than two weeks have passed.)

The tests can also be used for diagnosis when a doctor suspects a child has multisystem inflammatory syndrome, a rare but serious complication of Covid-19 in children. Because it is not known how long after the initial infection this inflammation begins, doctors should do both a PCR test and an antibody test, the guidelines said.

The Centers for Disease Control and Prevention also issued guidelines for using antibody tests; they were last updated on Aug. 1. The guidance is based on validations of test kits by the National Cancer Institute, while the infectious disease society reviewed test results from all over the world, said Dr. Kimberly E. Hanson, a testing expert at the University of Utah Hospital who was also on the expert panel.

The panel found it hard to decide which tests were worth recommending.

The Food and Drug Administration’s decision in April to approve tests without even reviewing their safety and efficacy data “turned out to be a problem,” Dr. Hanson said. “The market was flooded with poorly performing tests.”

After harsh criticism from testing experts and Congress, the F.D.A. reversed that decision in May and gave test-makers 10 days to prove their tests were accurate or be banned from selling them.

On Wednesday, the Department of Health and Human Services, citing an executive order from President Trump, said that laboratories that developed their own in-house tests for the virus did not have to pass F.D.A. review. The order does not apply to the commercial tests the panel analyzed.