Antigen testing offers an alternative testing method that directly detects the SARS-CoV-2 virus that causes COVID-19. While nucleic acid amplification testing (NAAT) or polymerase chain reaction (PCR) testing detects the virus’ genetic material (RNA), antigen testing detects a protein present on the virus. Antigen and RNA tests are the preferred methods to detect a current infection as opposed to blood tests for antibodies which should not be used to diagnose a current infection. CDC offers general guidelines for the use of these tests which are summarized below. Specific guidelines are available for use of this testing in nursing homes as well.
Antigen testing is performed with a swab of the nose. These tests are less expensive and available as point of care tests, meaning they can return results quickly without needing to be sent to a lab. Making results more readily available allows for faster public health action to restrict transmission of the virus. The speed at which these tests can provide results and reduced cost of testing provide distinct advantages.
A disadvantage of antigen testing is it may be less sensitive than PCR testing, meaning it may be less likely to detect the virus when it is actually present. Antigen testing is most effective in identifying the virus in those with symptoms in the early stages of their infection. It also has a role in testing close contacts to known cases who may be more likely to be infected. Screening with antigen testing is primarily recommended for congregate settings such as nursing homes. Currently in South Carolina, nursing homes are using this type of testing to detect and control the spread of the virus.
Knowing that all testing may return either false positive or false negative results, those getting tested should work with their medical provider to understand the results of their test. Medical providers should know the characteristics and recommendations related to the specific tests they are using and interpret results in the context of the specific patient’s circumstances (clinical and exposure history) which may affect the pre-test probability and thus the accuracy of the test. Some general guidance on interpreting results:
1. Positive antigen test in patient with symptoms or suspected to have been a close contact to a COVID-19 case
- No follow up testing recommended, treat as a true positive.
2. Positive antigen test in someone with no symptoms or not known to be a close contact (antigen testing not currently recommended in this situation aside from screening in congregate settings)
- Treat as a true positive (patient should isolate) but could consider follow up testing the PCR within two (2) days of the original test. A negative result on the PCR test may suggest a false positive.
- If the PCR test is performed more than two (2) days after the antigen test, it should be considered a separate test, not a follow up, and does not impact the original positive antigen result.
3. Negative antigen test in someone with no symptoms and not a close contact
- Treat as a negative result and no follow up test recommended.
4. Negative antigen test in someone with symptoms and/or a known close contact
- Patients with symptoms should continue to isolate and follow up PCR testing is recommended.
- A patient in quarantine after a close contact must complete their full quarantine period even if they test negative.
A patient who tests positive by PCR or antigen testing does not need additional testing for 90 days after the initial test if they recover and remain asymptomatic. Those receiving a positive antigen result should consider themselves as having COVID-19 and should begin isolating at home (or continue if they had been instructed to do so pending the test results).
All COVID-19 positive results should be reported to the appropriate DHEC regional office by phone within 24 hours. All COVID-19 test results, including positive, negative and indeterminate results, must be submitted to DHEC using an approved reporting method. This includes results of point of care tests even if they do not generate a lab report. Providers using this type of testing should work with DHEC to ensure they have an appropriate reporting plan in place. To inquire about electronic reporting options, please contact SCIONHELP@dhec.sc.gov. For details on disease reporting in South Carolina, see the South Carolina List of Reportable Conditions.