There are different types of tests. Molecular tests, or PCR tests, make many copies of RNA virus; these are the most sensitive tests. Antigen tests give answers more quickly, but are less accurate. Because you need more virus to see a positive result on an antigen test, there is a smaller window of time when an antigen test will be positive. Put another way, it is more likely that you will have a false negative antigen test, than a false negative PCR test. Finally, antibody tests detect proteins in the blood that are made in response to viral infection, telling you if you previously had an infection at some point in the past.