Antigen testing is commonly used in the diagnosis of respiratory pathogens, including influenza virus and respiratory syncytial virus. The U.S. Food and Drug Administration (FDA) has granted an Emergency Use Authorization (EUA) for antigen testing that identifies SARS-CoV-2.
Antigen testing is an immunoassay that detects the presence of a specific viral antigen, which implies current viral infection. Antigen testing is currently authorized for nasopharyngeal or nasal swab specimens placed directly into the test extraction buffer or reagent. Antigen testing currently authorized includes bedside testing, laboratory testing, and self-testing for people of any age.
Antigen testing is relatively inexpensive and most can be used at the point of care. Most currently licensed tests return results in approximately 15-30 minutes. antigen testing for SARS-CoV-2 is generally less sensitive than real-time reverse transcription polymerase chain reaction (RT-PCR) and other nucleic acid amplification assays (NAAT) that detect the presence of viral nucleic acids.
Nasopharyngeal Swabs
However, NAAT can remain positive for weeks to months after initial infection and can detect levels of viral nucleic acid even when the virus cannot be cultured, suggesting that the presence of viral nucleic acid may not always indicate infectivity.
Proper interpretation of antigen test results and NAAT (when needed) is important for accurate clinical management of patients or persons suspected of having COVID-19, or for identifying infected persons at screening.
The clinical performance of diagnostic tests depends heavily on the setting in which they are used. Both antigen tests and NAATs perform best if people are tested when viral loads are typically highest. Because antigen testing works best in symptomatic people and for a specific number of days after the onset of symptoms, antigen testing is often used in symptomatic people. Antigen testing may also be informative in diagnostic testing situations, where the person is known to have had contact with a COVID-19 patient.
Nasal Swabs
The accumulation of data on the performance of antigen tests in different situations can help guide the use of these tests as screening tests in asymptomatic populations to detect or rule out SARS-CoV-2 infection.
Antigen testing has been used to screen for high-risk tests in congregate housing settings, such as nursing homes, where repeat testing has rapidly identified patients with COVID-19, informing infection prevention and control measures and thus preventing transmission. In such cases, where rapid test turnaround time is critical, there is value in providing immediate results through antigen testing, even though they may be less sensitive than NAAT.
Healthcare providers and public health practitioners should be aware of test performance characteristics to identify potential false-negative or false-positive test results and to guide additional confirmatory testing and administration to patients or individuals.
As described in this guidance, laboratories and testing professionals who perform antigen testing should be aware of the factors that affect the accuracy of antigen testing. Healthcare providers, laboratory and testing professionals, and public health practitioners should also be aware of the differences between diagnostic, screening, and surveillance tests.
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