https://www.mayoclinic.org/diseases-conditions/coronavirus/expert-answers/covid-antibody-tests/faq-20484429
When is antibody testing done and why is it important?
Antibody testing, also known as serology testing, is usually done after full recovery from COVID-19. Eligibility may vary, depending on the availability of tests. A health care professional takes a blood sample, usually by a finger p*ick or by drawing blood from a vein in the arm. Then the sample is tested to determine whether you've developed antibodies against the virus. The immune system produces these antibodies — proteins that are critical for fighting and clearing out the virus.
If test results show that you have antibodies, it indicates that you were likely infected with COVID-19 at some time in the past. It may also mean that you have some immunity. But there's a lack of evidence on whether having antibodies means you're protected against reinfection with COVID-19. The level of immunity and how long immunity lasts are not yet known. Ongoing studies will eventually reveal more data on this.
The timing and type of antibody test affects accuracy. If you have testing too early in the course of infection, when the immune response is still building up in your body, the test may not detect antibodies. So antibody testing is not recommended until at least 14 days after the onset of symptoms. The U.S. Food and Drug Administration (FDA) authorized specific antibody tests, but tests with questionable accuracy are still on the market.
Another benefit of accurate antibody testing is that people who've recovered from COVID-19 may be eligible to donate plasma, a part of their blood. This plasma could be used to treat others with severe disease and boost the ability to fight the virus. Doctors call this convalescent plasma.