Several common rapid antigen tests work well for Omicron, according to a new study.

According to a new field study that raises concerns about possible false negative test results.

The tests performed similarly for Omicron and the Delta variant in the study, which was released Monday but has not yet been published in a peer-reviewed journal. Of the people who tested positive for the virus on a PCR test, 61 percent of those with Omicron infections also tested positive for a rapid antigen test within 48 hours, compared with 46 percent of those with Delta infections, according to the study, a collaboration between the National Institutes of Health, the Food and Drug Administration and the UMass Chan Medical School. The difference between variants was not statistically significant.

The tests performed better in people with the highest viral loads, detecting more than 90 percent of Omicron and Delta infections in this group, the researchers found.

“This study adds to the evidence that says Omicron can be detected with the home tests we have,” said Nathaniel Hafer, a molecular biologist at UMass Medical School and an author of the study.

Rapid antigen tests, which are less sensitive than PCR tests, are designed to detect proteins on the surface of the virus. If genetic mutations alter these proteins, it could affect the tests’ ability to detect the virus. So every time a new variant pops up, researchers have to reevaluate the tests.

Early lab research suggested that some antigen tests may be less sensitive to detecting Omicron than earlier variants, meaning they could generate more false negatives. The FDA warned of that possibility in late December.

But experts had noted that the tests had yet to be evaluated in large, real-world studies.

The new findings come from an ongoing US study that began in October and was designed to assess the performance of rapid antigen testing in asymptomatic people.

Participants received home collection PCR sets and one of three randomly assigned brands of rapid antigen testing by mail. They collected PCR samples and performed a rapid antigen test every 48 hours for 15 days. They sent their PCR samples to a lab for testing and reported the results of their rapid antigen testing in a research app. (They were also asked to upload photos of their rapid test results.)

Between October and the end of January, nearly 6,000 people took part in the survey. The new analysis focuses on 153 people who tested positive for the virus on a PCR test at least once during that period. About sixty percent had confirmed or probable Omicron infections, the researchers concluded, using a combination of sequence data and information about when each person first tested positive. The rest were believed to have Delta.

The PCR results suggested that about half of the 153 participants had a high viral load. Of this group, 96 percent of those with Omicron infections and 91 percent of those with Delta infections tested positive for an antigen test within two days of their positive PCR result.

“The study showed that when there are higher amounts of the virus, these antigen tests will work well in detecting cases,” said Matthew Binnicker, the director of clinical virology at Mayo Clinic, who was not involved in the study. “The real concern of false negatives is when there are lower levels of the virus.”

Experts urge people who have symptoms of or have been exposed to the virus to undergo multiple antigen tests, over a period of several days, to increase the chances of detecting an infection.

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