But in recent weeks, as cases have surged in many states, the demand for testing has soared, surpassing capacity and creating a new testing crisis.
In many cities, officials said a combination of factors was now fueling the problem: a shortage of certain supplies, backlogs at laboratories that process the tests, and skyrocketing growth of the virus as cases climb in almost 40 states.
Fast, widely available testing is crucial to controlling the virus over the long term, experts say, particularly as the country reopens. With a virus that can spread through asymptomatic people, screening large numbers of people is seen as essential to identifying those who are carrying the virus.
As states with major outbreaks struggled to keep up with demand for testing, the Department of Health and Human Services announced on Tuesday a new model of “surge testing” to help particularly hard-hit areas keep up.
At eight temporary sites in three cities — Baton Rouge, La., Edinburg, Texas, and Jacksonville, Fla. — the federal government will offer 15,000 free tests every day, for as many as 12 days. Results will likely be reported within five days, H.H.S. said.
The sites will allow wide access: Anyone over the age of five can be tested, including anyone with symptoms or possible exposure to a confirmed case, or anyone worried about being infected. Those tested do not need to live in the cities with the testing sites.
The effort was a sign of the Trump administration’s new concern about local resources at a time when it has largely handed off responsibility to states. But on a telephone briefing with reporters Tuesday afternoon, Adm. Brett P. Giroir, the assistant secretary for health, said that the new sites were “not to substitute for state testing,” and that there were no plans for the federal government to extend its stays.