The coronavirus test kits are small and fast — they produce results in as little as 15 minutes — and when they were first distributed to nursing homes around the country in August by the federal government, they were welcomed with open arms.

At last, it seemed, there was a solution to the delays and equipment shortages that had stymied efforts to use laboratory-based tests to curb outbreaks.

But now Nevada has ordered its nursing facilities to immediately suspend the use of two of the rapid virus tests after their performance was found to be lacking, according to a directive issued by the state’s department of health.

The order was prompted by a spate of false-positive results, in which the tests mistakenly found that healthy people were infected. The state directed that use of the kits be discontinued “until the accuracy of the tests can be further evaluated,” the Nevada document said.

The rapid tests are manufactured by two companies: Quidel, and Becton, Dickinson and Company, Representatives for the companies defended their products and said they were conducting investigations into the reports of false positives in Nevada.

Lisa Sanders, director of media relations at LeadingAge, an association of nonprofit providers of aging services, said that several nursing homes in other states had been experiencing issues with BD and Quidel’s tests and reporting them to her organization and the American Health Care Association in recent weeks.

In submitting their applications to the Food and Drug Administration for emergency clearance, both BD and Quidel declared that their tests had no false positives.

But shortly after the tests were rolled out across the state this summer, nursing homes began to report that people who had been evaluated by both the rapid tests and a slower but highly reliable laboratory test, called P.C.R., were getting conflicting results. Among 39 positive antigen test results from both BD and Quidel, 23 were found by P.C.R. to be negative — an error rate of nearly 60 percent.

The results, which were collected from a dozen facilities where thousands of tests had been performed, prompted the state to pivot away from antigen tests like BD’s and Quintel’s to viral RNA tests such as P.C.R., according to the directive.

Susan Butler-Wu, a clinical microbiologist at the University of Southern California, said the findings in Nevada could be emblematic of a larger issue: the use of tests in ways for which they were not designed or validated.

Both BD’s and Quidel’s tests received F.D.A. clearance for use “within the first five days of the onset of symptoms.” The instructions that come with BD’s test have noted that “the performance of this test has not been evaluated for use in patients without signs and symptoms of respiratory infection and performance may differ in asymptomatic individuals.”

Shannon Litz, a spokeswoman for Nevada’s department of health and human services, said in an email that the agency would be re-evaluating the tests’ performance before resuming their use.