https://www.webmd.com/lung/antibody-testing-covid-19#1

How Does It Work?

You’ll have to give some blood, like through a finger prick. Doctors test for two kinds of antibodies to SARS-CoV-2. They look for:

IgM antibodies, which develop early in an infection, and

IgG antibodies, which are more likely to show up later after you’ve recovered

It takes your body about 4 weeks to develop IgM antibodies. But scientists aren’t sure how long it’ll take for this to happen with SARS-CoV-2. More tests are needed to find out.

Keep in mind that current antibody tests can’t tell you if you’re immune to COVID-19. That's because we don't know how long these antibodies might protect you against the coronavirus. And these tests shouldn’t be used to diagnose the virus.

The National Institutes of Health (NIH) wants to study blood from 10,000 healthy people who haven’t tested positive for COVID-19. They want to see how many people might have had the virus without knowing it. If you want to take part, email the NIH at clinicalstudiesunit@nih.gov.

The CDC, along with other private and public labs, is working to develop more tests for the public. Right now, the FDA has given emergency authorization to only one antibody test. But the agency gave the OK for more than 70 companies to sell their tests. That means these tests aren’t FDA-approved, but the manufacturers promise to follow certain rules when making them.