Do you have antibody against Covid-19 virus?

 There are many tests to answer this question.

"Some test only for a transient antibody IgM that spikes while the body is in the throes of an active infection. An antibody IgG that peaks about four weeks after infection and typically marks longer-term immunity is a separate target. There are tests that look for both antibodies; others also look for a third involved in respiratory infections.

The most reliable ones involve a laboratory technique called Elisa that can indicate the amount of antibodies a person may have. Researchers are not certain that people who have recovered from Covid-19 have immunity, though it would hopefully follow the usual pattern for viral infections. Higher levels of antibodies generally mean a stronger physiological response, but it is unclear what levels are needed for immunity to the new coronavirus — or how long it might last.

Most of the tests offered are rapid tests that can be assessed in a doctor’s office — or, eventually, even at home — and provide simple yes-or-no results. Makers of the tests have aggressively marketed them to businesses and doctors, and thousands of Americans have already taken them, costing a patient roughly $60 to $115.

Rapid tests are by far the easiest to administer. But they are also the most unreliable — so much so that the World Health Organization recommends against their use.

Germany, which has emerged as a model among Western democracies in its efforts to curb the spread of the virus, is pursuing one of the most ambitious antibody studies, striving to test its entire population. It is a leader in the technology, has made its own antibody tests and conducted extensive diagnostic screening from the beginning.

The American F.D.A. has received requests for emergency-use authorization from 120 antibody-testing developers. So far it has granted formal approval to just four: Cellex, Ortho Clinical Diagnostics, Chembio Diagnostic Systems and the Mount Sinai Laboratory.

Less than 5 percent of the U.S. population may be infected, and even in hot zones like New York or New Orleans, the prevalence may not be higher than 10 to 15 percent, according to Dr. Osterholm. In China, early screening in hard-hit Wuhan indicates that only about 3 percent of the population has antibodies against the new coronavirus.

When the proportion of people exposed is that low, the tests’ false positive rate — signaling antibodies where there are none — can limit the tests’ utility.

Even Cellex’s F.D.A.-authorized test has a false positive rate of about 5 percent. That is still a significant margin of error: In a community where 5 percent of people have had the virus, Dr. Osterholm said, there would be as many false positives as true ones."