FLAGSTAFF, Ariz. (AP) – Access to health care for American Indians is difficult to gauge because the agency that oversees it does a poor job of tracking patient wait times, a report by a federal watchdog found.

Long wait times are a known problem at hospitals and health centers run by the Indian Health Service, particularly in rural areas where unemployment and poverty levels are high, the U.S. Government Accountability Office said Thursday. New patients waited up to four months to see a physician at a facility on the Navajo Nation and up to a month for a routine vision check at a clinic in the Billings, Montana, region, staff told federal investigators.

But until the Indian Health Service develops a way to monitor patient wait times across all its facilities, it won’t be able to tell whether health care has improved for a population that disproportionately suffers from diseases like diabetes and chronic liver disease, the report said.

Officials with the Indian Health Service say they struggle to provide timely care because of staffing shortages and outdated equipment. Some of its hospitals are decades old and some equipment doesn’t work or is used long past its usual lifetime.

Messages left with the Indian Health Service and the U.S. Department of Health and Human Services were not immediately returned.

The Indian Health Service’s Great Plains region has been under intense scrutiny lately due to findings of woefully inadequate services at some of the facilities. Congressional hearings have focused on the quality of care for tribal members who are guaranteed free health care as a condition of treaties their governments signed with the United States.

U.S. Sen. John Barrasso of Wyoming, chairman of the Senate Committee on Indian Affairs, said the lack of oversight at the health facilities is not surprising but tribal members deserve better.

“IHS continually fails to put the needs of patients first, and the findings in this report are just another example of that,” he said in a statement Thursday.