Ellume, an Australian company that makes a widely available at-home coronavirus test, has recalled nearly 200,000 test kits because of concerns about a higher-than-expected rate of false positives. That represents about 5.6 percent of the approximately 3.5 million test kits Ellume has shipped to the United States.

The company, which detected the problem in mid-September, traced the issue to variations in the quality of one of the raw materials used in the test kit, Dr. Sean Parsons, Ellume’s chief executive, said in a phone interview. He declined to specify the material in question, citing a desire not to publicly disclose precisely how the test kits work.

Approximately 427,000 test kits, including some provided to the U.S. Department of Defense, were affected by the problem, Dr. Parsons said. Roughly half have already been used, he said, yielding about 42,000 positive results. As many as a quarter of those positives may have been inaccurate, Dr. Parsons said, although he stressed that it would be difficult to determine exactly how many.

“I’m very sorry that this has happened,” Dr. Parsons said. “We’re all about chasing accuracy, and to have these false positives is disappointing.”

The issue did not affect all Ellume test kits or the reliability of negative results, the company said.

Ellume’s test is a rapid antigen test, designed to detect pieces of the virus in the nose. Users swab their nostrils, insert the swab into a dropper of fluid and then add the fluid to a Bluetooth-connected analyzer. Results are transmitted to a smart phone app in 15 minutes. Last December, it became the first over-the-counter, completely at-home test to receive an emergency use authorization from the U.S. Food and Drug Administration.

The company has asked retailers to remove the tests from shelves and is in the process of notifying consumers, Dr. Parsons said.

Consumers who have one of the affected tests can request a replacement online. People who try to use one of the affected test kits will be notified in the app that the test has been recalled. “It really won’t be possible to use any of those tests now,” Dr. Parsons said.

He added that the company had put “extra controls” in place to prevent the same problem from cropping up again in the future.

“We are doing everything possible to get known, good product into the hands of consumers in the U.S.,” Dr. Parsons said.

The recall comes as demand for testing has soared, and consumers have complained that at-home test kits are hard to find.

On Monday, the F.D.A. authorized a new at-home antigen test, ACON Laboratories’ Flowflex. The authorization “is expected to double rapid at-home testing capacity in the U.S. over the next several weeks,” Dr. Jeffrey E. Shuren, who directs the F.D.A.’s Center for Devices and Radiological Health, said in a statement. “By year’s end, the manufacturer plans to produce more than 100 million tests per month, and this number will rise to 200 million per month by February 2022.”

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