"For the 47 million uninsured, it's a pretty capricious system -- this is essentially an unmanaged group -- a group of people who don't know what they're going to get," Dr. Saul Weiner of the University of Illinois at Chicago said in a statement.

"They are getting care from a set of institutions that are uncomfortable and unsure about what it is they want to provide."

The researchers analyzed how three medical centers with different ownership models rationed uncompensated care to patients without insurance. Data were obtained from hospital financial reports, a survey of 292 self-pay patients, and from the self-pay policies and practices of front-line hospital staff.

The study, published in the journal Medical Care, found that all three institutions had policies for accepting self-pay patients, but that many times their practices did not match the policies. Prepayment was often left to the discretion of front line staff, who also determined whether a patient might be turned away from care, the study said.