When patients are hospitalized more than once in the same month, it may have more to do with their income or education levels than the quality of care they received, a U.S. study suggests.

Perhaps unsurprisingly, patients 85 and older are more likely to return to the hospital within 30 days of being sent home than people a decade or two younger, according to the analysis of data from Medicare, the U.S. health program for the elderly and disabled.

But patients also have higher odds of returning soon after discharge if they lack a high school diploma, have limited income and assets or have health benefits from Medicaid, the U.S. health program for the poor.

The findings suggest that Medicare penalties for what's known as readmissions under the Affordable Care Act may in some instances mete out punishment for outcomes that are beyond doctors' control, said lead study author Dr. Michael Barnett and senior author Dr. Michael McWilliams, colleagues at Harvard Medical School and Brigham and Women's Hospital in Boston.

"Hospitals are being penalized to a large extent based on the patients they serve," the doctors said by email. "Patients admitted to hospitals with higher readmission rates are sicker and more socially disadvantaged in a variety of ways than patients admitted to hospitals with lower readmission rates."

Under the current penalty system, Medicare deducts 3 percent from inpatient payments to hospitals with higher than expected readmission rates, the researchers report in JAMA Internal Medicine. Expected rates are only adjusted for patients' age, sex and recent diagnoses including the one from their hospital stay.

In 2014, the second year of the program, about 2,600 hospitals were fined a combined $428 million for excessive readmissions, the authors report.

To get a better understanding of how individual patient characteristics might influence repeat hospitalizations, the researchers examined several other variables Medicare doesn't consider in determining expected readmission rates - such as education and income levels, marital status, employment, race and ethnicity, smoking status and drinking habits.

They linked records from a nationwide health and retirement survey of Americans over 50 collected between 2000 and 2010 to data from Medicare claims from 2000 to 2012. The combined analysis assessed more than 8,000 hospital admissions.

The researchers sorted hospitals into quintiles based on readmission rates. They found that at least half of the observed difference in the probability of repeat hospitalizations between hospitals with the highest and lowest readmission rates might be accounted for by patient characteristics not currently considered by Medicare.

When researchers only used Medicare's criteria comparing readmission rates, they found the probability of repeat hospitalization was about 15 percent at facilities with the lowest rates and about 19.5 percent at hospitals with the highest rates.

But when they took another look using more criteria on patients' medical, social and economic characteristics, the gap between hospitals with the lowest and the highest readmission rates narrowed to 16 percent and 18.4 percent, respectively, odds of repeat hospitalization.

One limitation of the study, the authors acknowledge, is the data didn't allow them to calculate how considering individual patient characteristics might impact readmission rates at specific hospitals.

Even so, the findings suggest that the current Medicare penalty system for repeat hospitalizations may put facilities serving poor communities at a distinct financial disadvantage, Dr. Carl van Walraven, a senior scientist at the Ottawa Hospital Research Institute in Canada, noted in an accompanying editorial.

"Differences between hospitals in readmissions may be due to who is treated rather than how they're treated," van Walraven said by email.

SOURCE: http://bit.ly/1MmPcWX and http://bit.ly/1UT2grE JAMA Internal Medicine, online September 14, 2015.