Antibody tests for COVID-19 are currently available. Northwestern University in Chicago and Mt Sinai School of Medicine in New York have used SARS-CoV-2 IgG antibody tests in their research to identify people who have either had or were exposed to the virus and recovered. Results from Northwestern's SCAN research studies have already been published..These tests have been developed to be very specific for COVID-19, although there is always a very remote chance that they may detect previous infections with other similar coronaviruses.The basic science of antibodies is that once you are infected, your antibody levels will go up soon after infection and gradually decline over time. The immune system has memory cells that will reactivate if you are exposed to the same (or possibly similar) virus or infection again, and it should again produce antibodies to protect you. Antibody tests for Covid-19 are also available from commercial labs such at Quest Diagnostics. No, they shouldn't be used to diaginose an active infection, but they can be useful to tell someone whether they've had the virus and developed an antibody response to it. The problem is, you have to test at the right time. You could show negative if you test too soon after infection, or if if you wait too long after having recovered, because your antibodies may have already declined to the point of nondetection. Depending on the antibody test used and the lab that processes it, they will report the results as qualitative (ie, the antibody is "present or not present") or quantitative (whereby the actual level of the antibody is reported).