A cell, in greenish brown, heavily infected with coronavirus particles, in pink.

Such a test may help scientists learn how widespread the infection is, and how long people remain immune after recovering.

A cell, in greenish brown, heavily infected with coronavirus particles, in pink.Credit…Niaid, via Reuters

The Food and Drug Administration on Thursday approved a new test for coronavirus antibodies, the first for use in the United States.

Currently available tests are designed to find fragments of viral genes indicating an ongoing infection. Doctors swab the nose and throat, and amplify any genetic material from the virus found there.

 The new test, by contrast, looks for protective antibodies in a finger prick of blood. It tells doctors whether a patient has ever been exposed to the virus and now may have some immunity.

That is important for several reasons. People with immunity might be able to venture safely from their homes and help shore up the work force. It may be particularly important for doctors and nurses to know whether they have antibodies.

Antibody testing eventually should give scientists a better sense of how widespread the infection is in the population — and help researchers calculate more precisely the death rate.

“If we don’t know the asymptomatic or mild cases, we won’t know if it’s killing a sizable fraction of the people who have it, or only people who have underlying conditions or are very unlucky,” said Dr. Carl Bergstrom, an infectious diseases expert at the University of Washington in Seattle.

Antibody tests already are used in China, Singapore and a handful of other countries. Public Health England has purchased millions of antibody tests and hopes to make them available for home use in the United Kingdom.

The new test, made by Cellex, looks for two types of antibodies: immunoglobulin M, made by the body a few days into an infection; and immunoglobulin G, made later but created specifically to neutralize a particular invader.

The test delivers results in about 15 minutes. But just having antibodies does not guarantee immunity from the coronavirus.

The new test looks only for the presence of the antibodies and delivers a qualitative yes or no answer — it does not say how well any antibodies are working, said Dr. Angela Rasmussen, a virologist at Columbia University in New York.

Some people who are infected produce powerful immune responses; others do not. And scientists do not know how long immunity to this virus may last.

Although the new test is approved for diagnosis, antibody tests are not likely to be positive in the early days of an infection. New evidence suggests that people are most contagious two to three days before they have symptoms.

“In those cases, it’s hard to say how good this test would be,” Dr. Rasmussen said. “This would probably be most useful for rapid testing of people in the clinic who are symptomatic, and not so much for asymptomatic or presymptomatic people.”

Traditional PCR tests, with swabbing of the nose and throat, are more likely to detect a new coronavirus infection.

Via NYTimes.com