Unlike tests to diagnose diseases, antibody tests show who has been infected and recovered. The body makes antibodies in response to many illnesses and infections, including other coronaviruses. New blood tests are being developed to identify antibodies unique to SARS-CoV-2, the official name of the new coronavirus. The tests look for two kinds of antibodies: immunoglobulin M (IgM) and G (IgG). The body quickly produces IgM antibodies for its initial attack against infections. It makes IgG antibodies more slowly and retains them longer; IgG antibodies suggest possible immunity. Some companies are developing finger-prick tests that get results in minutes. Others are developing far more accurate tests called enzyme-linked immunosorbent assays (ELISA) that require sending blood samples to a lab for analysis. “I would argue that in the present circumstance, sensitivity, specificity and accuracy are more important than convenience,” said Dr. James Zehnder, director of clinical pathology at Stanford Medicine in California. Labs across the country are developing antibody tests that  could be widely available within weeks, said Dr. Elitza Theel, director of the Infectious Diseases Serology Laboratory at the Mayo Clinic in Rochester, Minnesota. It is not yet clear how many tests will be needed - but that number could run into the hundreds of millions - or when supplies will be adequate. More than 60 vendors have notified the U.S. Food and Drug Administration of plans to release antibody tests, Theel said. The FDA will allow release of some tests without standard review if developers determine they are accurate. Abbott Laboratories (ABT.N) launched a lab-based IgG antibody test on Wednesday without going through FDA review and is expecting to ship nearly one million tests this week alone. Dozens of manufacturers are also seeking FDA approval for antibody tests under an accelerated emergency review process. Earlier this month, Cellex Inc became the first company to win emergency approval for an ELISA test. BioMedomics Inc and Becton Dickinson and Co (BDX.N) have launched a point-of-care finger-prick test and said they expect to supply more than one million tests over the coming months. As tests flood the market, accuracy will vary, Theel warned. “We have to make sure the test we are using is giving us the most accurate results,” she said. An ELISA test for IgG antibodies being developed at Mayo Clinic correctly identifies who has coronavirus antibodies more than 95% of the time, Theel said. Antibody tests can help calculate what portion of the population has already been infected, as well as whether infections were mild or severe. Governments and companies could use antibody tests to determine who would most likely be safe to return to work and public interactions, and whether it is safe to lift stay-at-home orders all at once in some regions or in stages based on infection risk. People with negative antibody tests or very low antibody levels would likely have higher risk of infection than people with high antibody levels. While antibodies to many infectious diseases typically confer some level of immunity, whether that is the case with this unique coronavirus is not yet known. And how strong immunity might be, or how long it might last in people previously infected, is not clear. With some diseases like measles the immunity can be lifelong. With others, immunity can wane over time. “We cannot know with certainty that reinfection is not possible until further research is performed,” said Dr. David Reich, president and chief operating officer of The Mount Sinai Hospital in New York City.
Explainer: What are coronavirus antibody tests?.