- MS-DRG
- CMS relative weight
- MS-DRG CMI
- APR-DRG
- SOI subclass
- ROM subclass
01 · Working principle
The executive shorthand
Use the MS-DRG lane to discuss the applicable grouping, fiscal-year relative weights, eligible denominator, and CMS-style CMI. APR-DRG also has its own relative weights, alongside base groups and separately assigned SOI and ROM subclasses. An APR-based measure needs its own method, weight set, and population; it must not be blended with the MS-DRG measure. Always label the method and version.
- MS-DRG → fiscal-year relative weight → eligible-discharge average → CMI.
- APR-DRG → base APR group → SOI subclass and ROM subclass.
- Never substitute an APR severity value for an MS-DRG relative weight.
02 · Working principle
Why the distinction changes the review
A finance question about MS-DRG weight movement and a quality question about APR risk-of-mortality distribution may involve the same encounters but not the same calculation. Put them beside each other with shared cohort information; do not force them into one number.
Executive checklist
What leaders should ask next
- Which grouper, version, and effective period produced each classification?
- Is this view discussing payment weight, severity, mortality risk, or all three separately?
- Are missing and ungroupable encounters visible?
- Does any calculation improperly mix the two methods?
Interpretation boundary
Limits to keep visible
- CMI is an aggregate of applicable DRG relative weights, not a complete measure of patient severity or hospital performance.
- MS-DRG and APR-DRG should not be combined into one synthetic metric.
- Classification and payment references change; use the appropriate effective version rather than automatically using the newest file.
Authoritative references
Sources and method notes
References reviewed September 28, 2026. They support the definitions and governance context; the operating-review guidance is sanalytics’ editorial interpretation, not an endorsed industry standard.
- Centers for Medicare & Medicaid Services: Case Mix Index
CMS definition of hospital CMI using Medicare discharges; the page distinguishes transfer-adjusted and unadjusted calculations.
- Centers for Medicare & Medicaid Services: MS-DRG Classifications and Software
Official MS-DRG classifications, software, and effective periods, including midyear grouper updates. Use the version applicable to the discharge date.
- Centers for Medicare & Medicaid Services: FY 2026 IPPS Final Rule Home Page
FY 2026 Table 5 includes MS-DRG relative weights and geometric and arithmetic mean length of stay. It is a named reference year, not a recommendation to use FY 2026 for every cohort.
- Agency for Healthcare Research and Quality: APR-DRG Severity of Illness
APR-DRG severity-of-illness context and four-level subclass structure.
- Solventum: APR-DRG Classification System
The methodology owner describes APR-DRG relative weights, separate four-level SOI and ROM subclasses, and the need for weights appropriate to the population.