The U.S. Food and Drug Administration (FDA) is alerting clinical laboratory staff and health care providers that false positive results can occur with antigen tests, including when users do not follow the instructions for use of antigen tests for the rapid detection of SARS-CoV-2. Generally, antigen tests are indicated for the qualitative (relating to, measuring, or measured by the quality of something rather than its quantity) detection of SARS-CoV-2 antigens in authorized specimen types collected from individuals who are suspected of COVID-19 by their healthcare provider within a certain number of days of symptom onset. The FDA is aware of reports of false positive results associated with antigen tests used in nursing homes and other settings and continues to monitor and evaluate these reports and other available information about device safety and performance.
The FDA reminds clinical laboratory staff and health care providers about the risk of false positive results with all laboratory tests. (Including the PCR no doubt)
Laboratories should expect some false positive results to occur even when very accurate tests are used for screening large populations with a low prevalence of infection. Health care providers and clinical laboratory staff can help ensure accurate reporting of test results by following the authorized instructions for use of a test and key steps in the testing process as recommended by the Centers for Disease Control and Prevention (CDC), including routine follow-up testing (reflex testing) with a molecular assay when appropriate, and by considering the expected occurrence of false positive results when interpreting test results in their patient populations.
The FDA recommends clinical laboratory staff and health care providers who use antigen tests for the rapid detection of SARS-CoV-2:
- Be aware that the Conditions of Authorization in the antigen
Emergency Use Authorizations specify that authorized laboratories are to
follow the manufacturer's instructions for use, typically found in the
package insert, when performing the test and reading test results. If
you no longer have the package insert for the test you are using, you
can contact the manufacturer. The authorized instructions for use for
each test can also be found on the FDA's COVID-19 IVD EUA webpage.
- For example, the package insert for tests include instructions for handling of the test cartridge/card, such as ensuring it is not stored open prior to use. If the test components are not stored properly, this can affect the performance of the test.
- The package insert for tests also includes instructions about reading the test results, including the appropriate time to read the results. Reading the test before or after the specified time could result in false positive or false negative results.
- Be aware that processing multiple specimens in batch mode may make it more challenging to ensure the correct incubation time for each specimen. Refer to the package insert and ensure proper timing for each specimen when processing the specimen in the test device and reading the results.
- Be careful to minimize the risks of cross-contamination when testing patient specimens, which can cause false positive results. Insufficient cleaning of the workspace, insufficient disinfection of the instrument, or inappropriate use of protective equipment (for example, failing to change gloves between patients) can increase the risk of cross-contamination between specimens with subsequent false positive results. Consider the CDC guidance for changing gloves and cleaning work area between specimen handling and processing.
- Consider the CDC's recommendations when using antigen testing in nursing homes and other settings. For positive results, especially in low incidence counties, consider performing confirmatory RT-PCR test within 48 hours.
- Remember that positive predictive value (PPV) varies with disease
prevalence when interpreting results from diagnostic tests. PPV is the
percent of positive test results that are true positives. As disease
prevalence decreases, the percent of test results that are false
- For example, a test with 98% specificity would have a PPV of just over 80% in a population with 10% prevalence, meaning 20 out of 100 positive results would be false positives.
- The same test would only have a PPV of approximately 30% in a population with 1% prevalence, meaning 70 out of 100 positive results would be false positives. (which informs us that only 30 percent percent of the results would be correct) This means that, in a population with 1% prevalence, only 30% of individuals with positive test results actually have the disease.
- At 0.1% prevalence, the PPV would only be 4%, meaning that 96 out of 100 positive results would be false positives.
- Health care providers should take the local prevalence into consideration when interpreting diagnostic test results.
- Consider positive results in combination with clinical observations, patient history, and epidemiological information.
- Be aware that the Conditions of Authorization in the antigen EUAs specify that Authorized Laboratories are to collect information on the performance of antigen tests and report any suspected occurrence of false positive or false negative results and significant deviations from the established performance characteristics of which they become aware to both the FDA and the test manufacturer.
Are laboratories exercising due diligence?