Emergency rooms account for about half of the nation’s hospital admissions and accounted for virtually all of the rise in admissions between 2003 and 2009, according to a study released on Monday.

Although emergency rooms are widely considered expensive places for diagnostic care, physicians are increasingly relying on them to determine whether a patient needs to be hospitalized.

The study’s findings raise important questions about how emergency rooms contribute to high health care costs in the United States and what their role will be in the future as the nation undergoes fundamental changes in health care delivery. One goal of the Obama administration’s health care law was to reduce reliance on costly emergency room care.

The report by the RAND Corporation, a nonprofit research group, was done for a consortium of emergency medicine physician organizations. RAND, which says it was chosen for its independence, says the sponsors had no say in the report’s findings.