Case mix index is calculated as:
Total MS-DRG relative weights ÷ Total inpatient discharges
Therefore:
4,375 ÷ 2,500 = 1.75
The hospital's CMI is 1.75.
CMI provides an aggregate indication of the average relative resource intensity of the hospital's inpatient population. Because each MS-DRG carries a relative weight, hospitals treating larger numbers of complex medical or surgical cases typically have higher CMIs than hospitals treating less resource-intensive populations.
CMI is not a direct quality score and should not be interpreted as a pure measure of CDI performance. It can change because of service-line mix, surgical volume, patient acuity, transfers, annual MS-DRG recalibration, coding accuracy, documentation quality, and other operational changes.
CMS maintains the FY 2026 MS-DRG Definitions Manual and grouper logic, currently Version 43.1 for April 1 through September 30, 2026.
A CDI manager should therefore analyze unexpected CMI changes alongside service-line trends, query activity, coding reconciliation, procedure volumes, and diagnosis patterns.
CCDS Reference Topics: CMI; relative weights; CDI analytics; MS-DRGs; trend analysis.
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