Advertisements and news stories about blood antibody tests to detect past COVID-19 infection are becoming more common. Blood antibody tests for COVID-19 are different than the tests done with nose or throat swabs. Swab tests can help determine who is at risk for developing symptoms of COVID-19 or spreading the disease by detecting the virus in the person’s respiratory system. Blood tests, however, tests for past viral infections by testing not for the virus itself, but for the antibodies a person’s body produced to fight the virus. And, importantly, not all antibody tests are reliable. In this assessment, the MAT highlights some of the differences between these two types of tests.