Coronavirus Rapid Testing Falters in People Without Symptoms: Study

Mr. Bryant, of Quidel, who received an early copy of the University of Arizona study, praised the results as “very, very good,” citing the Sofia’s ability to root out “people who are infectious.”

Quidel, which is running several studies of its own, does not yet have definitive results that show the Sofia works in people without Covid-19 symptoms. But “based on the data so far, it seems to be applicable to that population,” Mr. Bryant said of his company’s results.

Other experts advised caution.

Although C.T. values do tend to increase as virus levels diminish, exceptions to this trend exist — and there is no universal “magic-number cutoff” for infectiousness, Dr. Dien Bard said.

Failing to grow the coronavirus out of a person’s sample also does not guarantee that individual is not contagious to others, said Omai Garner, the associate director of clinical microbiology in the UCLA Health System, who was not involved in the study.

Several experts noted that the University of Arizona study did not track transmission among its participants, making it impossible to draw conclusions about how, and from whom, the virus spread.

Dr. Harris said that some of the concerns about the Sofia’s accuracy could be overcome with repeat testing. Screened frequently enough with a rapid test, infected people missed by one Sofia would probably be detected with the next, especially if the levels of virus in their bodies were rising, Dr. Harris said. He and his colleagues are now gathering data on University of Arizona athletes, who are tested daily, to investigate this possibility. Quidel has also partnered with the Pac-12 and Big Ten Conferences to conduct daily tests.

People with symptoms or known exposures to the coronavirus should still get the most precise and reliable tests available — those that use P.C.R., said Susan Butler-Wu, a clinical microbiologist at the University of Southern California who was not involved in the study. More data, she added, would be needed to figure out how rapid tests fit into the larger diagnostic landscape.