CT scans are far more expensive, they expose patients to a low dose of radiation, and the Centers for Disease Control and Prevention and some medical groups recommend against using them to diagnose Covid-19. But they were widely used in China to identify cases, and their reliability there is fueling growing interest in adding chest CT to the diagnostic arsenal in the U.S.

The scans detect hazy, patchy, “ground glass” white spots in the lung, a telltale sign of Covid-19. In one recent study of 1,014 patients, published in the journal Radiology, scientists in China reported that chest CT found 97% of Covid-19 infections. In comparison, the study found that 48% of patients who had negative results on the swab test, which detect the coronavirus’s genome, in fact had the disease.

“Once you’re a couple of days into infection, chest CT scans don’t miss,” said an emergency medicine physician in Louisiana who asked not to named. With the swab test missing 30% to 50% of cases, physicians in China called for the diagnostic use of CT early in the outbreak there, and “fever clinics” set up in Wuhan and elsewhere began routinely using them.