The case mix index measures the average clinical complexity of a hospital’s inpatients. Calculate CMI from MS-DRG relative weights and discharges, see which DRGs drive it, and estimate DRG payments and CMI-adjusted cost.
How it works
Calculate the case mix index from MS-DRG relative weights and discharges, see which DRGs drive acuity, and estimate DRG payments and CMI-adjusted cost per discharge.
Case mix index = Σ (Relative weight × Discharges) ÷ Σ Discharges
Expected DRG payment = Base rate × Σ Relative weights
CMI-adjusted cost per discharge = Cost ÷ Discharges ÷ CMICommon uses
- Monitoring patient acuity and documentation quality
- Adjusting cost and revenue benchmarks for patient complexity
- DRG payment and service-line planning
Worked example
The calculator above opens with this example. Change any input to see the results update instantly, or select Reset to return to these figures.
| DRGs (illustrative weights) | 871 Septicemia w MCC | 1.9 | 420 291 Heart failure w MCC | 1.3 | 310 470 Major joint replacement | 1.9 | 260 193 Pneumonia w MCC | 1.3 | 240 392 GI disorders w/o MCC | 0.8 | 380 690 Kidney & UTI w/o MCC | 0.8 | 290 247 Coronary stent w/o MCC | 2.1 | 140 |
| Blended base payment rate (optional) | $6,800 |
| Case mix index | 1.3907 |
| Total discharges | 2,040 |
| Total relative weight | 2,837.00 |
| Largest weight contributor | 871 Septicemia w MCC |
| Expected DRG payments | $19,291,600 |
Each 0.01 change in CMI changes DRG payments by roughly 0.01 ÷ CMI in percentage terms, holding volume constant. Sample weights are illustrative; replace them with current-year MS-DRG weights.
Interpreting the results
- A rising CMI can reflect sicker patients, new service lines or better clinical documentation and coding.
- Compare CMI only within the same weight set; CMS recalibrates MS-DRG weights every fiscal year.
- Track Medicare and all-payer CMI separately, since payer populations differ in acuity.
Frequently asked questions
How is the case mix index calculated?
The case mix index equals the sum of MS-DRG relative weights for all discharges divided by the number of discharges. A CMI of 1.50 means the average patient is 50% more resource-intensive than the national average case.
Why did our case mix index change?
CMI changes with patient acuity, the mix of service lines, and the completeness of clinical documentation and coding. It also changes when CMS recalibrates MS-DRG weights at the start of each federal fiscal year.
Where can I find current MS-DRG relative weights?
CMS publishes the MS-DRG relative weights each year in Table 5 of the Inpatient Prospective Payment System final rule. Use the weights for the fiscal year in which the discharges occurred.
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