The F.D.A. in September told makers of rapid tests that they would be required to continue to test their products as new variants emerged, and, if asked, to share those results with the agency.
Many companies have announced that their tests can detect Omicron, and several independent scientists said that they believed the tests should be capable of recognizing the variant, especially when present at high levels. But a few new studies raise questions about the tests’ sensitivity.
The F.D.A.’s update stemmed from an evaluation it is doing with the National Institutes of Health, said Bruce Tromberg, director of the National Institute of Biomedical Imaging and Bioengineering. The scientists evaluated the antigen tests using pooled samples of nasal mucus collected from multiple people with Omicron, as well as similar pooled samples from Delta patients.
Scientists then diluted each of these pooled samples until the antigen tests no longer detected the virus. The tests were able to detect more dilute solutions of Delta samples than Omicron samples, suggesting that the tests may be less sensitive to the new variant, Dr. Tromberg said. Still, he added, in real-world settings, “it may not translate into any different sensitivity.”
An F.D.A. spokeswoman, Stephanie Caccomo, said last week that studies were underway “to confirm the reason for the apparent decreased sensitivity.”
“Once that is known,” she said, “adjustments to existing tests can be undertaken by each developer with support from the F.D.A., if appropriate.”
The F.D.A. update was not the first hint that some antigen tests may be less sensitive to Omicron. Dr. Eckerle and her colleagues recently evaluated the performance of seven antigen tests against samples of the virus grown from specimens taken from people infected with Omicron. Overall, the researchers found, the tests were less sensitive to Omicron than to previous variants.
“They missed samples with infectious virus, and they missed samples that had quite a decent viral load,” Dr. Eckerle said. The work has not yet been published in a scientific journal.