PCR vs. Antigen Testing

• Antigen testing, unlike molecular testing, does not involve any amplification.  Thus, it is not nearly as sensitive and able to pick up low levels of virus as compared to molecular tests.
• Since antigen testing is dependent upon higher levels of the virus for detection, it is most accurate in the acute phase of infection when viral loads are the highest.  All current FDA EUA antigen tests (n=4) are only authorized for use from symptom onset up to 5-12 days past symptom onset due to this.  (The number of days varies by assay.)  Molecular tests do not have this limitation due to their sensitivity and limit of detection.
• Molecular RT-PCR tests remain the gold standard by the FDA and CDC for the clinical diagnostic detection of SARS-CoV-2.  The CDC states it “may be necessary to confirm a rapid antigen test result with a nucleic acid tests, especially if the result of the antigen test is inconsistent with the clinical context”. https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antigen-tests-guidelines.html
• Current antigen tests are all based on nasal swabs, and CRL’s Rapid Response test is saliva.