Rapid antigen tests are commonly used in the diagnosis of respiratory illnesses. In this case, the rapid antigen detection test looks for proteins produced by the SARS-CoV-2 virus, which is the virus that causes the disease called COVID-19.
Antigen tests are immunoassays that detect the presence of a specific viral antigen, which means they identify people who currently have a viral infection. In some situations, they may be a viable alternative to nucleic acid amplification tests (NAAT or PCR tests), which look for viral RNA in the sample.