Antigen tests directly detect fragments of SARS-CoV-2 viral protein (as opposed to viral RNA detected by nucleic acid amplification tests [NAAT aka PCR tests]).
The main advantages of antigen tests are fast turnaround time, simple to perform point-of-care use, identification of current viral infection, and lower cost.
The main drawbacks are lower sensitivity (more false negatives) and intended use limited to individuals suspected of COVID-19 within 5-12 days of symptom onset (number of days varies by manufacturer).
Positive results in symptomatic individuals are likely to reflect an active infection, but negative tests are presumptive negative and depending on level of clinical concern may need to be confirmed with a NAAT/PCR.
A negative antigen test may not exclude infection and is of insufficient sensitivity to make decisions about discontinuing isolation.
Antigen tests are approved by the FDA for use on symptomatic individuals. No antigen test has yet been yet approved by the FDA for use on asymptomatic individuals.
If an antigen test is used outside the recommended window from symptom onset or to test asymptomatic individuals, false positive results can occur. Depending on the level of tolerance for potential false positive results, confirmatory NAAT may be indicated.
1) Quidel Sofia SARS Antigen FIA assay
2) BD Veritor System for Rapid Detection of SARS-CoV-2
3) LumiraDx SARS-CoV-2 Antigen Test
4) Abbott BinaxNOW COVID-19 Ag CARD
Sensitivity and specificity are measures of a test's ability to correctly classify a person as having a disease or not having a disease.
Sensitivity refers to a test's ability to designate an individual with disease as positive. A highly sensitive test means that there are few false negative results, and thus fewer cases of disease are missed.
Specificity refers to a test's ability to designate an individual who does not have a disease as negative. A highly specific test means that there are few false positive results.
Examples of scenarios in which SARS-CoV-2 antigen tests may reasonably be used:
Facilities that perform only CLIA-waived tests must possess either a CLIA Certificate of Waiver or a Certificate of Compliance. These locations may include long-term care facilities, hospitals, prisons, physician offices, urgent care clinics, outreach clinics, pharmacies and temporary patient care settings. Persons performing the point-of-care test must be trained to perform the test, but these sites have no routine regulatory testing oversight and no testing personnel requirements. If a large volume of tests is anticipated, such as in a skilled nursing facility, this may require full-time staff during weeks when response driven testing is performed.
CDPH will update this guidance as more data become available.