A popular at-home test detects most infectious Omicron cases, a study suggests.

The Abbott BinaxNOW, a widely used at-home coronavirus test, can detect the vast majority of people who are infected with the new Omicron variant and are carrying high levels of the virus, according to a new, real-world study of more than 700 people who visited a walk-up testing site in San Francisco.

The BinaxNOW is a rapid antigen test, which is designed to detect proteins on the outside of the coronavirus. Like all rapid antigen tests, it is less sensitive than P.C.R. tests, which can find even very small traces of the virus. But rapid antigen tests have generally been good at catching people who have high viral loads and are most likely to be infectious, although federal health officials have recently sent mixed messages about their usefulness.

The new study, which has not yet been reviewed by experts, found that overall, the BinaxNOW detected 65 percent of the infections identified via P.C.R. testing, but 95 percent of people who had the highest viral loads. Among those who had high levels of the virus, the tests caught 98 percent of those with symptoms and 90 percent of those without symptoms, the researchers found.

The test’s performance was roughly on par with what the scientists had observed in prior real-world studies, before Omicron emerged, they said.

“It’s working as it was designed,” said Joseph DeRisi, a biochemist at the University of California, San Francisco, and co-president of the Chan Zuckerberg Biohub and an author of the paper. “There does not seem to be any performance deficit with Omicron.”

The findings come after some preliminary studies raised questions about whether rapid antigen tests might be less sensitive for Omicron than for other variants — and whether saliva samples might be a better way to detect the variant than the nasal swabs currently used in antigen tests.

At a Senate hearing on the federal coronavirus response on Tuesday, Dr. Janet Woodcock, the acting commissioner of the Food and Drug Administration, said that scientists were still trying to determine how well the currently authorized at-home tests work for the new variant.

“We believe all of them detect Omicron,” she said. “We simply feel they are somewhat less sensitive than they were to some of the previous variants.”

The new research, which relied on nasal swabs, does not address how early in the course of an infection the antigen tests can detect the variant, as one small previous study did, or whether the tests would perform better with saliva samples. And the results cannot be extrapolated to other antigen tests, which need to be evaluated individually, the researchers cautioned.

But they suggest that one widely used test — the BinaxNOW — should still be able to pick up most infectious Omicron cases.

“This tool continues to be very important,” said Dr. Diane Havlir, an infectious disease physician specialist at the University of California, San Francisco, and an author of the paper.

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Around the world. The W.H.O. warned that more than half of people in Europe could be infected with Omicron in the next six to eight weeks. In Britain, Prime Minister Boris Johnson is again accused of violating his own lockdown rules after hosting a party at 10 Downing Street.

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The new study was conducted on Jan. 3 and 4 at a free San Francisco testing site. Lab assistants collected two nasal swabs from each of 731 people. The researchers tested one swab from each person with the BinaxNOW test and the other using P.C.R. tests.

Forty-one percent of those people tested positive for the virus on the P.C.R. tests. When they analyzed a subset of these positive samples, they found that 98.5 percent of them were Omicron.

Although the antigen test did miss a small proportion of people carrying high levels of the virus, most of those who tested positive on the P.C.R. test but not on the antigen test had low viral loads, Dr. DeRisi said. They may have been either in the earliest stage of infection or recovering from their infections by the time they were tested.

Because the tests may not catch people who have been infected very recently, the researchers recommend that people who are exposed to the virus then test more than once over a period of a few days.

“People can be on the upswing of the virus, and we want to make sure those people are detected,” Dr. Havlir said.

The study was performed independently of Abbott, she noted, although the company did provide BinaxNOW test kits for a prior study, in 2020. The test kits for the new study were provided by the health department, she said.

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