Joe Patterson, Ph.D.:
It’s important to remember what an antibody test is and isn’t, and how it works. An antibody test is not a test to determine if the individual currently has COVID-19, but rather, can give insight into if the individual has had COVID-19.
In an antibody test, a small amount of blood from the individual is used in the assay [an assay is an analysis or test]. The blood will be added to a substrate or media that contains a COVID-19 specific antigen or target for the antibody to bind to. This is followed by a chemical reaction with the bound antibody to determine if the individual has developed antibodies against COVID-19 due to a previous infection.
Antibody tests “can” be accurate. For the test to work properly, the person being tested has to have antibodies present at a sufficient level. The test also needs to be specific enough for antibodies against COVID-19, but not so specific that natural variation in how antibodies are produced from person to person cause a false negative. Unfortunately, there are many antibody tests out there that have received emergency use authorization, but are not very accurate.
Regarding immunity to future COVID-19 infections, more data needs to be collected. Every now and then, you hear a story of a person that has tested positive for COVID-19 twice, months apart. Whether this is really the case, or one of the tests was a false positive or lingering infection at a low titer is present is hard to say. There is just a lot that we still need to learn.
For more about antibodies and antibody testing, see the questions linked here.