As the number of coronavirus cases in the United States exceeds 9.2 million, experts continue to call for a massive scale-up of testing among both the healthy and the sick — a necessary measure, they have said, to curb the spread of an infection that can move swiftly and silently through the population.

One strategy has involved the widespread use of rapid tests, which forgo sophisticated equipment and can return results in minutes. Purchased in bulk by the federal government and shipped nationwide, millions of these products have already found their way into clinics, nursing homes, schools, athletic teams’ facilities and more, buoying hopes that the tests might hasten a return to normalcy.

But a new study casts doubt on whether rapid tests perform as promised under real-world conditions, especially when used in people without symptoms.