Assessment of strategies for managing expansion of diagnosis coding using risk-adjustment methods for medicare data

Since the passage of the Affordable Care Act (ACA) in 2010, many studies have used national Medicare data to examine associations between national hospital pay-for-performance programs and quality and costs of care. In January 2011, as the ACA was being implemented, the Centers for Medicare & Medicaid Services increased the number of available diagnosis billing codes from a maximum of 9 diagnosis codes (the primary diagnosis plus 8 comorbidities; a tenth code was reserved for coding external causes of injury and usually left blank) to 25 diagnosis codes (the primary diagnosis plus 24 comorbidities).