Case mix index is calculated by dividing the sum of MS-DRG relative weights by the number of inpatient discharges:
2,640 รท 1,600 = 1.65
Therefore, the CMI is 1.65.
CMI represents the average relative resource intensity of the hospital's inpatient cases. It is affected by the types of patients treated, surgical versus medical mix, tertiary services, transplant or trauma volume, documentation specificity, coding accuracy, and annual changes to MS-DRG relative weights and grouper logic.
CMS publishes hospital case-mix information and FY 2026 relative weighting factors through the IPPS Final Rule supporting data. Table 5 contains MS-DRGs and relative weighting factors, while Table 2 contains hospital-specific case-mix information.
A decline in CMI should not automatically be interpreted as a CDI failure. A shift toward lower-complexity cases or changes in service lines can legitimately reduce CMI even with excellent documentation.
CDI managers should therefore trend CMI alongside diagnosis capture, surgical volume, service lines, payer population, coding reconciliation, and quality indicators.
CCDS Reference Topics: Case mix index; MS-DRG relative weight; CDI analytics; hospital performance.
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