Expect false positives when interpreting rapid antigen test results, the Food and Drug Administration told long-term care providers and clinical lab workers Tuesday.
As are molecular tests, antigen tests typically are highly specific for the SARS-CoV-2 virus, the letter states. “However, all diagnostic tests may be subject to false positive results, especially in low prevalence settings. Healthcare providers should always carefully consider diagnostic test results in the context of all available clinical, diagnostic and epidemiological information.”
Various factors may contribute to a false positive finding, the agency explained. Users may not follow manufacturer instructions or specimen contamination can occur, for example. Patient-specific factors, including presence of human antibodies such as rheumatoid factor, and even the viscosity of the specimen can affect results.
The letter offers recommendations for administering the tests and interpreting results. It explains in detail what to expect as disease prevalence decreases. To ensure accurate reporting and interpretation, care providers should follow key steps in the testing process as recommended by the Centers for Disease Control and Prevention, it states.
Meanwhile, FDA officials continue to “monitor and evaluate these reports and other available information about device safety and performance,” the agency said.
The FDA’s message echoes that of health officials from other federal agencies, as they seek to reassure long-term care providers about the usefulness of these tests in the fight against COVID-19 outbreaks in their facilities.
Industry skepticism about the tests became apparent to all in early October when the state of Nevada’s health department temporarily banned their use in nursing homes after fielding operator complaints.
Although rapid antigen tests are not a perfect instrument, they are critical to keeping the coronavirus in check as cases trend back upward this season, said Adm. Brett Giroir, M.D., assistant secretary for health at the Department of Health and Human Services, in a recent press briefing.