The FDA, along with researchers from the National Institutes of Health, used samples from patients confirmed to be infected with the omicron variant to study how well antigen tests work. The agency said early results suggest that antigen tests “do detect the omicron variant but may have reduced sensitivity,” meaning it's possible the tests could miss an infection, known as a "false negative."
Case numbers are surging across the country, prompting high demand for at-home rapid testing. Antigen tests can be done at home, with results in minutes, while PCR tests are processed in labs and have longer turnaround times.
The FDA didn't say people should avoid using antigen tests.
"The FDA continues to authorize the use of these tests as directed in the authorized labeling and individuals should continue to use them in accordance with the instructions included with the tests," the agency said in a statement. "Antigen tests are generally less sensitive and less likely to pick up very early infections compared to molecular tests."
The FDA didn't specify the brands of tests used in the study.
Gigi Gronvall, a senior researcher at the Johns Hopkins Center for Health Security, said in a phone interview that the FDA's findings underscore the challenge posed by the evolving nature of the virus.
“With with every new variant, we have to make sure that the tests work, and it looks like the tests work for this. But they're not picking up infections as early as previous versions,” she said. “So if you feel any symptoms but test negative, it's not a get-out-of-jail-free card. You may need to test again and hold off a little bit on assuming that you're negative.”
The FDA encouraged follow-up testing Tuesday for anyone who tests negative with a rapid antigen test but is experiencing symptoms.
“If a person tests positive with an antigen test, they should self-isolate and seek follow-up care with a health care provider to determine the next steps,” the agency said.
President Joe Biden announced this month that the administration would distribute 500 million rapid Covid tests to U.S. households for free starting in January. The federal government also plans to set up new testing sites nationwide, in addition to the 20,000 already in operation.
The Centers for Disease Control and Prevention issued new guidance Monday saying people who test positive for Covid-19 but have no symptoms should isolate for five days, down from the previous recommendation of 10 days. It attributed the change to growing evidence that the virus is most infectious in the two or three days after symptoms arise.