FDA: Rapid Antigen Tests May Be Less Effective at Detecting Omicron

Rapid antigen tests may be less sensitive at detecting the highly contagious omicron variant and could lead to results that are "false negative," the Food and Drug Administration said Tuesday.

Preliminary research by the FDA, in collaboration with the National Institutes of Health's Rapid Acceleration of Diagnostics program, used samples from patients confirmed to be infected with the omicron strain of the virus to study the performance of at-home tests, also known as "antigen" tests.

The agency said early data suggests that antigen tests "do detect the omicron variant but may have reduced sensitivity," meaning it's possible such tests could miss an infection.

Previous testing by RADx focused on heat-inactivated virus samples, which are patient specimens confirmed to be infected with the omicron variant that have been heat-treated so that the virus is no longer live. Currently-available antigen tests were able to detect the mutated strain "with similar performance" to other variants in that study. But the agency notes that live virus is the best way to evaluate true test performance, while "heat-inactivated samples are the best available option when patient samples with live virus are not available."

The FDA stopped short of telling people to avoid using antigen tests and did not specify the brands of tests used in the study. The agency also emphasized the findings are based on laboratory data and not the more reliable clinical study evaluations, which are currently ongoing.

"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 new findings come as case numbers are surging across the country at a time when family and friends, seeking to gather over the holidays, have clamored for over-the-counter antigen tests to diagnose infections due to long lines for PCR tests — which are better at detecting low levels of the virus — and delayed turnaround times for their lab results.

But, the CDC said Wednesday that fewer than 1% of the U.S. COVID-19 cases genetically sequenced last week were the omicron variant; the delta variant accounted for more than 99%, reducing concerns about currently authorized antigen tests missing an infection. It comes a day after the agency significantly slashed estimates of the percentage of new cases linked to the omicron variant.

The CDC had previously reported the omicron variant comprised 73% of all cases for the week of Dec. 18. But on Tuesday, the agency revised that number to 22.5%, suggesting that while the new variant was on the rise, delta — which is the more severe strain — is still a driving force behind the current surge. The most recent report estimates that as of Dec. 25, 59% of all U.S. infections were caused by omicron, a figure that will likely be amended after more data from genetic sequencing is processed.

Rapid antigen tests, which work by detecting surface proteins of the coronavirus, are relatively inexpensive and quick, with results known in around 15 minutes. They can, however, miss the early stages of COVID-19 infection. Most popular at-home tests advise users to take two tests on separate days to ensure more accurate results.

In an interview with NBC's "TODAY" show, CDC Director Dr. Rochelle Walensky said while antigen tests may not perform as well as they did for other variants, "it's still picking up quite a bit of infection" and the agency encourages their use.

"They are a really really helpful tool," Walensky said. "But what we would reiterate, and is also in our guidance, is that if you have a negative antigen test and you have symptoms, then you should go ahead and get that PCR test. And those antigen tests are really helpful for things like testing to stay in school, where we’re getting an antigen test every day, or every other day, or where they’re using them in higher education to screen students every several days, that’s where they perform really quite well, as well."

The FDA recommends that if a person tests negative on an antigen test but has symptoms of COVID-19 or has a “high likelihood of infection due to exposure,” they should follow-up with a molecular PCR test to confirm the result. A person who tests positive with an antigen test, should self-isolate and seek follow-up care with a health care provider to determine the next steps, the agency said.

Earlier this month, the White House announced the Biden administration would distribute 500 million rapid COVID-19 tests to U.S. households for free starting in January.