U.S. Department of Health & Human Services

06/14/2021 | Press release | Distributed by Public on 06/14/2021 09:45

Interim Guidance for Rapid Antigen Testing for SARS-CoV-2

Summary of Recent Changes

  • Updated guidance based on new published studies on antigen test performance.
  • Clarification about which NAATs should be used for confirmatory testing.
  • Considerations for people who have had previous SARS-CoV-2 infections and those who have been fully vaccinated.
  • Two new antigen testing algorithms, one for congregate living settings, and one for community settings.
  • Updates to testing suggestions for fully vaccinated, asymptomatic people.

    View Previous Updates

Key Points

  • This interim guidance is intended for healthcare providers who order antigen tests, receive antigen test results, or perform point-of-care testing, as well as for laboratory professionals who perform antigen testing in a laboratory setting or at the point of care and report those results.
  • The purpose of this interim technical guidance is to support effective clinical and public health use of antigen tests for different testing situations.
  • This guidance applies to all clinical and consumer uses of antigen tests and is not specific to any particular age group.
  • This guidance incorporates considerations for fully vaccinated people and should be used in conjunction with CDC's Interim Public Health Recommendations for Fully Vaccinated People.

Antigen Testing for SARS-CoV-2

General Guidance

Antigen tests are commonly used in the diagnosis of respiratory pathogens, including influenza viruses and respiratory syncytial virus. 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 EUAsexternal icon .

Antigen tests are immunoassays that detect the presence of a specific viral antigen, which implies current viral infection. Antigen tests are currently authorized to be performed on nasopharyngeal or nasal swab specimens placed directly into the assay's extraction buffer or reagent. The currently authorized antigen tests include point-of-care, laboratory-based, and self-tests, and they are applicable to people of any age. See Table 1 for additional information about antigen tests.

Antigen tests are relatively inexpensive, and most can be used at the point of care. Most of the currently authorized tests return results in approximately 15-30 minutes. 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) for detecting the presence of viral nucleic acid. However, NAATs can remain positive for weeks to months after initial infection and can detect levels of viral nucleic acid even when virus cannot be cultured, suggesting that the presence of viral nucleic acid may not always indicate contagiousness.

Proper interpretation of both antigen test results and NAATs (when indicated) is important for accurate clinical management of patients or people with suspected COVID-19, or for identification of infected people when used for screening.

The clinical performance of diagnostic tests largely depends on the circumstances in which they are used. Both antigen tests and NAATs perform best if the person is tested when their viral load is generally highest. Because antigen tests perform best in symptomatic people and within a certain number of days since symptom onset, antigen tests are used frequently on people who are symptomatic. Antigen tests also may be informative in diagnostic testing situations in which the person has a known exposure to a person with COVID-19.

Accumulation of data on the performance of antigen tests in different situations has helped guide the use of these tests as screening tests in asymptomatic people to detect or exclude SARS-CoV-2 infection. See FDA's Recommendations for healthcare providers using SARS-CoV-2 diagnostic tests for screening asymptomatic individuals for COVID-19external icon . Also see information from the Centers for Medicare & Medicaid Services (CMS) on Updated CLIA SARS-CoV-2 Molecular and Antigen Point of Care Test Enforcement Discretionexternal icon .

Antigen tests have been used for screening testing in high-risk congregate housing settings, such as nursing homes, in which repeat testing has quickly identified people with COVID-19, informing infection prevention and control measures, thus preventing transmission. In this case, and where rapid test turnaround time is critical, there is value in providing immediate results with antigen tests, even though they may have lower sensitivity than NAATs.

Healthcare providers and public health practitioners should understand test performance characteristics to recognize potentially false negative or false positive test results and to guide additional confirmatory testing and management of the patient or person. Laboratory and testing professionals who perform antigen tests should understand the factors that affect the accuracy of antigen testing, as described in this guidance. Healthcare providers, laboratory and testing professionals, and public health practitioners should also understand the differences among diagnostic, screening, and surveillance testing. See CDC's Overview of Testing for SARS-CoV-2, and Testing Strategies for SARS-CoV-2. Also see FDA's FAQs on Testing for SARS-CoV-2external icon .

This guidance supplements and is consistent with CDC's Overview of Testing for SARS-CoV-2 and SARS-CoV-2 Point-of-Care and Rapid Testing guidance. CDC has also published guidance on SARS-CoV-2 Antigen Testing in Long Term Care Facilities, Interim Guidance for SARS-CoV-2 Testing in Correctional and Detention Facilities, Interim Guidance for SARS-CoV-2 Testing in Homeless Shelters and Encampments, and Operating Schools During COVID-19: CDC's Considerations.

Regulatory Requirements for Using Antigen Tests for SARS-CoV-2

FDA regulates in vitro diagnostic devices and has provided recommendations and information regarding EUA requests for COVID-19 diagnostic tests in the Policy for Coronavirus Disease-2019 Tests During the Public Health Emergency (Revised) ('Policy for COVID-19 Tests')external icon and the EUA templates referenced in that policy. COVID-19 tests and test systems used for diagnostic or screening testing, including those for antigen testing, must have received an EUA from FDA or be offered under the policies in FDA's Policy for COVID-19 Testsexternal icon . Every antigen test for SARS-CoV-2 authorized for use by FDA is included on FDA's list of In Vitro Diagnostics EUAsexternal icon . The intended use of each test, available in the Instructions for Use and in the Letter of Authorization, defines the population in which the test is intended to be used, the acceptable specimen types, and how the results should be used.

Laboratory and testing professionals who conduct diagnostic or screening testing for SARS-CoV-2 with antigen tests must also comply with Clinical Laboratory Improvement Amendments (CLIA) regulations. Any laboratory or testing site that intends to report patient-specific test results to a person or healthcare provider must first obtain a CLIA certificate and meet all requirements to perform that testing. For more information, see CMS' How to Obtain a CLIA Certificateexternal icon . CMS has provided additional information on Enforcement discretion for the use of SARS-CoV-2 point-of-care testing on asymptomatic individualsexternal icon .

Performance of Antigen Tests for SARS-CoV-2

It is important for healthcare providers and testing personnel to understand the performance characteristics, including sensitivity, specificity, and positive and negative predictive values, of the particular antigen test being used, and to follow the manufacturer's instructions for use, which summarize performance characteristics. See FDA's In Vitro Diagnostics EUAsexternal icon 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. Thus, it may be necessary to confirm an antigen test result with a laboratory-based NAAT, especially if the result of the antigen test is inconsistent with the clinical context. Table 1 summarizes the differences between NAATs and antigen tests. Clinical performance of NAATs and antigen tests may differ from clinical utility when considering issues of test availability, quality of specimen collection and transport, and turnaround times of results. Based on their instructions for use, some point-of-care NAATs may not be used for confirmatory testing. NAATs that generate presumptive results are not appropriate for use in confirmatory testing.

The sensitivity of antigen tests varies but is generally lower than most laboratory-based NAATs. The antigen level in specimens collected either before symptom onset, or late in the course of infection, may be below the tests' limit of detection. This may result in a negative antigen test result, while a more sensitive test, such as most NAATs, may return a positive result. Studiesexternal icon have shown that antigen tests have comparable sensitivity to laboratory-based NAATs when viral load in the specimen is high and the person is likely to be most contagious.

The specificity of antigen tests is generally as high as most 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 will occur, especially when used in communities where the prevalence of infection is low - a circumstance that is true for all in vitro diagnostic tests. In general, for all diagnostic tests, the lower the prevalence of infection in the community, the higher the proportion of false positive test results.

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 considers both the prevalence of the target infection in the population that is being tested as well as the clinical context of the individual being tested. If the prevalence of infection in the community is high, and the person being tested is symptomatic, then the pretest probability is generally considered high. If the prevalence of infection in the community is low, and the person being tested is asymptomatic and has not had any known contact to a person with COVID-19, then the pretest probability is generally considered low. See CDC's Interpreting Results of Diagnostic Tests for additional information on the relationship between pretest probability and the likelihood of positive and negative predictive values.

To help estimate pretest probability, CDC recommends that laboratory and testing professionals who perform antigen testing determine infection prevalence based on a rolling average of the positivity rate of their own SARS-CoV-2 testing over the previous 7-10 days. Alternatively, state health departments generally publish COVID-19 data on testing positivity rates and case rates for their communities.

Processing of Antigen Tests for SARS-CoV-2

The Conditions of Authorization in the antigen EUAs specify that CLIA-certified laboratories and testing sites are to follow the manufacturer's instructions for use, typically found in the package insert, when performing the test and reading test results. The authorized instructions for use for each test can also be found at FDA's In Vitro Diagnostics EUAsexternal icon .

For example, the performance of antigen tests can be affected if the test components are not stored and handled properly. They should never be frozen and should always be allowed to reach room temperature (15-30°C) before use. The package insert for these tests includes instructions for handling of the test cartridge/card, such as ensuring it remains in its sealed pouch until immediately before use.

The package insert for antigen tests also includes instructions about how to read the test results, including the appropriate time to read the results and whether the results should be interpreted visually or with an instrument analyzer. Reading the test before or after the specified time could result in false positive or false negative test results.

Processing multiple specimens successively or in batch mode may increase the risk of contamination and may make it more challenging to ensure that each specimen is incubated for the correct amount of time before the result is read. Refer to the package insert for the correct incubation time for that test, and then monitor and ensure proper timing for each specimen during testing and when reading results.

All testing for SARS-CoV-2, including antigen testing, depends on the integrity of the specimen, which is affected by procedures for both specimen collection and handling. Improper specimen collection, such as swabbing the nostril too quickly, may cause insufficient specimen collection, resulting in limited amounts of viral genetic or antigenic material for detection. Time from specimen collection to testing should be minimized, and the temperature of the specimen during this time must be controlled. See CDC's Interim Guidelines for Collecting, Handling, and Testing Clinical Specimens for COVID-19.

Quality assurance procedures should be followed to prevent cross-contamination and inaccurate test results. For example, users should follow the manufacturer's instructions, as well as state and local guidance, for when and how often to perform testing on control specimens. If antigen testing returns multiple unexpected positive results, it may be appropriate to stop testing patient specimens, review all procedures, disinfect all surfaces, change gloves, and run control specimens before restarting the testing of patient specimens. In such circumstances, confirmatory testing should be considered for people who received unexpected results, regardless of pretest probabilities.

Decontaminate work surfaces and equipment with appropriate disinfectants by using an EPA-approved disinfectant for SARS-CoV-2, following the manufacturer's recommendations for use, such as dilution, contact time, and safe handling. See EPA's List of Disinfectants for COVID-19external icon . Gloves should be changed before collecting, handling, and processing a new specimen in the antigen test system. Failing to change gloves can increase the risk of cross-contamination and false antigen test results. See CDC's guidance on Point-of-Care Testing, and Interim Laboratory Biosafety Guidelines for Handling and Processing Specimens Associated with Coronavirus Disease 2019 (COVID-19).

Some antigen tests have explored the use of viral transport medium (VTM) during specimen collection, but the use of VTM may cause false test results from either cross-reactivity with the capture antibodies or dilution of the specimen that decreases the sensitivity of the test. Laboratories and testing sites should refer to the instructions for use and the package insert that are specific for the test that they are using regarding the use of VTM.

Also see FDA's Letter to Clinical Laboratory Staff and Health Care Providersexternal icon on the potential for false positive results with antigen tests, and CDC's guidance on Point-of-Care Testing.

Evaluating the Results of Antigen Testing for SARS-CoV-2

Evaluating the results of an antigen test for SARS-CoV-2 depends primarily on the clinical and epidemiological context of the person who has been tested (e.g., symptoms, exposure to others with COVID-19, vaccination status, previous infection status, or setting in which they live). For additional details on testing recommendations see guidance for fully vaccinated people. A particularly important aspect of epidemiological context is whether the person to be tested is a resident or an employee of a congregate living facility. In addition, evaluating the results of an antigen test for SARS-CoV-2 should consider the performance characteristics (e.g., sensitivity, specificity) and the instructions for use of the FDA-authorized test, and the prevalence of SARS-CoV-2 infection in that particular community (percent positivity rate over the previous 7-10 days or the number of cases in the community relative to the population size).

The evaluation of an antigen test result should consider whether the person has experienced symptoms, and if so for how long. Generally, healthcare providers can rely upon a positive antigen test result for a symptomatic patient because the specificity of current FDA-authorized antigen tests is high.

The sensitivity of current FDA-authorized antigen tests varies, and thus negative diagnostic testing results should be handled depending on the circumstances. In most circumstances, the manufacturers' instructions for use of antigen tests indicate that negative test results should be considered 'presumptive,' meaning that they are preliminary results. See FDA's In Vitro Diagnostics EUAsexternal icon .

It may be appropriate to confirm antigen test results with a laboratory-based NAAT. For confirmatory testing, CDC recommends using a laboratory-based NAAT that has been evaluated against the FDA reference panel for analytical sensitivity. See FDA's SARS-CoV-2 Reference Panel Comparative Dataexternal icon . NAATs that generate presumptive results are not appropriate for use in confirmatory testing.