The factors often blamed for poor hospital readmission rates—socioeconomic status, access to care and insurance coverage—do not account for why readmissions are higher at some hospitals than others or why Medicare-Medicaid dual eligibles are readmitted more often, new research shows.

Yale University researchers led a team that compared readmissions among patients diagnosed with three major acute conditions who covered by both Medicare and Medicaid to those with Medicare alone for an article published in JAMA Health Forum on Friday.

The findings offer a rebuttal to a common explanation of why some hospitals have more readmissions: that facilities treating a disproportionate share of patients with low incomes, poor health and other negative circumstances are at a disadvantage relative to their peers.

“Inequities in hospital readmission rates for dual-eligible patients are not the primary result of differences measurable across communities, highlighting that hospitals may have a distinct role in advancing equity for socioeconomically disadvantaged patients,” the authors wrote.

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Researchers from Yale, Mathematica Policy Research, Boston-based Brigham and Women’s Hospital, Vertex Pharmaceuticals and George Washington University first looked at the differences in readmissions between the dual-eligible population and those covered only by Medicare. The discrepancy was stark.

The study took into account Census data on three disparities that influence patient health: health services availability (like primary care density in a county), state Medicaid policies and enrollment, and socioeconomic factors such unemployment and lack of a transportation.

Readmission rates were then adjusted for 2.5 million patients treated at more than 4,000 hospitals from 2014 and 2017 based on those socioeconomic markers. There was a modest decrease in readmission rates after taking the factors into account. But if having more low socioeconomic patients really were to blame for higher readmissions, then there’d be no difference in the readmission rates for each set of patients.

Dual-eligible patients still had worse readmission rates than their counterparts, however. The authors conclude hospitals must close the gap. “Hospitals may reduce these internal disparities by targeting data collection and analytical resources, cultural transformation efforts and quality improvement activities on strengthening equity in outcomes,” the study says.