Rapid coronavirus antigen tests may give false positives, FDA warns

The Food and Drug Administration (FDA) warned about the possibility of false positives that can occur when using rapid antigen tests to detect coronavirus, particularly if the test is not used correctly. The regulatory agency said it has received reports of false-positive results occurring in nursing homes and other health care settings.

The agency warned that reading the test results either before or after the specified time provided in the instructions can result in false-positive or false-negative results. It also referenced the antigen EUA conditions of authorization, which specifies that authorized laboratories are to follow the manufacturer’s instructions for use regarding administering the test and reading the results.

Mishandling the test, or not ensuring that it’s been stored properly before use, could also result in a skewed result. Processing multiple specimens at once may also affect test results, as it may make it difficult to ensure the correct incubation time for each specimen.

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“Be careful to minimize the risks of cross-contamination when testing patient specimens, which can cause false-positive results,” the FDA warned. “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.”

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The agency also suggested the following Centers for Disease Control and Prevention recommendations when it comes to using antigen testing in nursing homes, and to consider performing confirmatory testing within 48 hours of a positive result.

“In general, antigen tests are not as sensitive as molecular tests,” the FDA wrote. “Due to the potential for decreased sensitivity compared to molecular assays, negative results from an antigen test may need to be confirmed with a molecular test prior to making treatment decisions. Negative results from an antigen test should be considered in the context of clinical observations, patient history and epidemiological information.”

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For its part, the FDA said it is working with test manufacturers to ensure that instructions for use are as clear as possible, and to safeguard COVID test use in nursing homes and other settings.