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Case Mix Index

The Case Mix Index (CMI) is a measure of the relative cost or resources needed to treat the mix of patients in each licensed California hospital during the calendar year. To calculate the CMI, OSHPD uses Medicare Severity-Diagnosis Related Groups (MS-DRG) and their associated weights, assigned to each MS-DRG by the Centers for Medicare & Medicaid Services (CMS). Each patient record is assigned an MS-DRG based on the principal and secondary diagnoses, age, procedures performed, the presence of co-morbidities and/or complications, discharge status, and gender. Each MS-DRG has a numeric weight reflecting the national "average hospital resource consumption" by patient for that MS-DRG, relative to the national "average hospital resource consumption" of all patients. Although the MS-DRG weights are based on resource consumption by Medicare patients, OSHPD applies them to all patient discharge data reported by hospitals in California during the course of a calendar year. Click here for an example of how the CMI is used and the method to calculate it.

On October 1, 2007 CMS replaced the then-current 538 DRGs with 745 MS-DRGs. OSHPD implemented these changes with the release of its 2008 Patient Discharge Data. The grouper software is updated by CMS at the beginning of each federal fiscal year (October 1st) and applied to patient records based on their reported discharge date. In the past, OSHPD adjusted the grouper and applied it to records based on a calendar year. With the implementation of MS-DRG, OSHPD now applies the grouper to patient records based on the federal fiscal year. MS-DRG Grouper version 25.0 was applied to discharges from January 1, 2008 through September 30, 2008. MS-DRG Grouper version 26.0 was applied to the discharges from October 1, 2008 through September 30, 2009 and so on for subsequent years. Because of this change, the CMI for each hospital is based on patient discharges within a given federal fiscal year beginning on October 1, 2008 and forward.

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