Readmission penalties and health insurance expansions: a dispatch from Massachusetts.

Background: Payers are penalizing hospitals for high readmission rates. It is unknown whether major changes in population insurance coverage can affect readmission rates, despite the Affordable Care Act's coverage expansions coming into effect this year.

Objective: To evaluate the impact of a large-scale insurance expansion on hospital readmissions, using Massachusetts' 2006 health reform as a natural experiment.