Pneumonia is a leading cause of death and hospitalization among U.S. adults, but hospitals and doctors are doing a poor job detecting its cause, according to a new study.

Most of the time doctors can't pinpoint the exact cause of the pneumonia, whether it is due to a virus or bacteria, according to the Centers for Disease Control and Prevention. The lung infection kills more than 53,000 a year, the CDC estimates.

The agency calls for quicker tests to rapidly identify the cause and promote better treatment.

The agency performed a two-and-a-half-year study that looked at community-acquired pneumonia hospitalization rates in three Chicago hospitals and two Nashville hospitals.

Researchers used chest x-rays and other diagnostic methods to determine pneumonia hospitalizations. Of the 2,488 adults studied, about 93 percent had pneumonia.

The problem was what caused it, and whether the pneumonia was due to a virus or bacteria.

Researchers detected viruses in 27 percent of patients and bacteria in 14 percent, CDC said. They did not find the exact cause in 62 percent of the patients, which worried the agency.

"This illustrates the need for more sensitive diagnostic methods that can both help guide treatment at the individual level as well as inform public health policy for adult pneumonia at a population level," said Dr. Seema Jain, lead author of the study and a CDC epidemiologist.