Antigen tests are immunoassays that detect the presence of a specific viral antigen, which suggests current viral infection. Antigen tests are commonly used in the diagnosis of respiratory pathogens, including influenza viruses and respiratory syncytial virus (RSV). The U.S. Food and Drug Administration (FDA) has granted emergency use authorization (EUA) for antigen tests that can identify SARS-CoV-2. See FDA’s list of In Vitro Diagnostics EUAs.
SARS-CoV-2 antigen tests are currently authorized for nasopharyngeal swab and nasal swab specimens. The currently authorized antigen tests include point-of-care (POC), laboratory-based, and self-tests available without a prescription. Certain tests have age limitations; refer to FDA’s website for more details.
Antigen tests produce results quickly (within minutes), and most can be used at the POC or at home. Most self-tests, or at-home tests, are antigen tests. Antigen tests are better at detecting a SARS-CoV-2 infection when someone has COVID-19 symptoms compared to if they do not. Antigen tests for SARS-CoV-2 are generally less sensitive than real-time reverse transcription polymerase chain reaction (RT-PCR) and other nucleic acid amplification tests (NAATs), which detect and amplify the presence of viral nucleic acid. For this reason, in situations where test sensitivity is of paramount importance, NAAT tests are preferred. For additional comparison between NAAT and antigen tests, please see the Summary Table of NAAT and Antigen Test Differences.
Accurate identification of infection and clinical management of COVID-19 requires performing the test properly and correctly interpreting the results. Patients who test positive should follow CDC isolation guidance for next steps. All initial negative antigen test results should be confirmed with a NAAT or repeated with additional antigen tests following FDA’s recommendations on repeat testing.
It is important for healthcare providers and testing professionals to understand the performance characteristics, including sensitivity, specificity, and positive and negative predictive values, of the antigen test being used, and to follow the manufacturer’s instructions for use, which summarize performance characteristics. See FDA’s In Vitro Diagnostics EUA for detailed information about specific authorized tests.
The “gold standard” for clinical diagnostic detection of SARS-CoV-2 remains laboratory-based (moderate- and high-complexity) NAATs. In situations where test sensitivity is of paramount importance, NAAT tests are preferred. Antigen tests do not have the same limits of detection as most NAATs, which have a higher sensitivity. Compared to NAATs, antigen tests are more likely to return a false negative, especially when testing before symptom onset when the level of antigens in a specimen is lower.
The specificity of antigen tests is comparable to NAATs, which means that false positive test results are unlikely when an antigen test is used according to the manufacturer’s instructions. Despite the high specificity of antigen tests, false positive results can occur, especially when used in situations where the pretest probability is low – a circumstance that is true for all in vitro diagnostic tests.
Positive and negative predictive values of all in vitro diagnostic tests (e.g., NAAT and antigen tests) vary depending upon the pretest probability. Pretest probability is the likelihood that the person being tested actually has the infection. Clinicians should consider the following when determining pretest probability for COVID-19: