Rapid antigen tests are commonly used in the diagnosis of respiratory pathogens including influenza viruses.  Rapid antigen tests perform best when the person is tested in the early stages of infection with SARS-CoV-2 when viral load is generally highest.  The FDA cites rapid antigen tests can be used for screening testing in settings in which repeat testing could quickly identify persons with SARS-CoV-2 infection to inform infection prevention and control measures, thus preventing transmission.  Further, in such setting, immediate results with antigen tests can be more valuable even though they may have lower sensitivity than PCR tests, especially when rapid turnaround time is required.
The table below highlights some key points of differentiation between PCR and antigen tests.
Summary of Some Differences between RT-PCR Tests and Antigen Tests
RT-PCR TestsAntigen Tests Intended UseDetect Current InfectionDetect Current Infection Analyte DetectedViral RNAViral Antigens Specimen Type(s)Nasal Swab, Sputum, SalivaNasal Swab SensitivityHighModerate SpecificityHighHigh Test ComplexityVariesRelatively Easy to Use Authorized for Use at the Point-of-CareMost devices are not, some devices areYes Turnaround TimeRanges from 15 minutes to > 2 daysApproximately 15 Minutes Cost/TestModerateLow
Source:  Interim Guidance for Rapid Antigen Testing for SARS-CoV2, September 4, 2020