A rapid COVID test is a diagnostic test. Virus antigens, which are molecules that tell your body to develop an immune response to an infection, are used to determine the results of rapid testing. Antigens are not the same as antibodies, which are produced by your immune system in response to antigen signals. This test can be administered by a trained healthcare provider in a number of settings.
For diagnosing an active infection, molecular or PCR testing are more reliable than antigen tests. When a PCR test is compared to a rapid antigen test, the PCR test can detect a single coronavirus RNA molecule in a patient sample. To acquire a positive result on a rapid antigen test, tens of thousands of molecules in the same size sample would be required.