Medicare used to give these hospitals extra payments for taking care of poor people. But under health care reform, they'll get less of that funding and more for how they perform on a variety of measurements, including how patients rate them.

"Our results suggest that safety-net hospitals are struggling on this important metric. As a result, safety-net hospitals are likely to get penalized under the new payment scheme," said Paula Chatterjee, a medical student at Harvard School of Public Health. "Given that safety-net hospitals are already financially stretched, even small losses can be potentially devastating for these hospitals."

At issue are hospitals that serve poor people, often including those who don't have health insurance. According to the study, they tend to be sicker than other patients and have less trust in the health care system.

Under health care reform, the federal government punishes hospitals that perform poorly on a variety of measures by not giving them some Medicare payments.