Case Mix Index (CMI)
A measure of the average complexity and resource intensity of a hospital's Medicare patients, calculated as the average MS-DRG relative weight across discharges.
On hospital cost-and-quality pages, Case Mix Index (CMI) carries a specific technical meaning that often differs from how the term is used in clinical practice or general medical writing. The definition here is the CMS-file usage. On the LakeQuality value rubric, Case Mix Index (CMI) is one of the inputs (directly or indirectly) to the combined cost-and-quality grade. Understanding how the term is computed at CMS — what counts and what does not — is part of reading hospital pages defensibly.
Each hospital page on LakeQuality surfaces the specific Case Mix Index (CMI) value for that facility (when CMS reports one), so the general definition here translates into a concrete data point on the per-hospital pages you actually use.
How It Works
CMI is the single number that summarizes how sick, on average, a hospital's Medicare inpatients are. It is calculated by summing the MS-DRG relative weights for every Medicare discharge during a period and dividing by the number of discharges. A CMI of 1.0 equals the national average; 2.0 means patients are, on average, twice as resource-intensive as the baseline. The national all-hospital CMI runs near 1.85 for FY2025. Large academic medical centers like Mass General Brigham, Cleveland Clinic main campus, UPMC Presbyterian, and UCSF typically run CMIs of 2.2-2.6 because they concentrate the most complex cases: transplants, ECMO, complex cardiac surgery, trauma. Community hospitals generally sit between 1.3 and 1.7. Critical access hospitals and small rural hospitals often run below 1.2 because they transfer complex cases out. CMI directly drives Medicare revenue: a hospital with 10,000 annual Medicare discharges at an average payment of $12,000 per standardized weight has total Medicare inpatient revenue of $120 million at CMI 1.0 and $240 million at CMI 2.0. This leverage makes CMI a compliance risk zone. The Recovery Audit Contractor (RAC) program and the Office of Inspector General (OIG) audit hospitals showing unexplained CMI growth. Hospitals that jump from 1.5 to 1.9 in a single year without a service-line change invite scrutiny for upcoding. Clinical documentation improvement (CDI) programs staffed by nurses and coders review physician documentation in near real time to ensure coded severity reflects actual clinical acuity, protecting both revenue and compliance.
Related Terms
- Diagnosis Related Group (DRG), A classification system that groups hospital inpatient stays into categories based on diagnosis, procedures, and patient complexity, used by Medicare to determine how much a hospital gets paid.
- MS-DRG (Medicare Severity DRG), The severity-adjusted version of the DRG system used by Medicare since 2007, expanding groupings to capture complication and comorbidity burden and refine payment accuracy.
- Inpatient Prospective Payment System (IPPS), Medicare's payment system for acute-care hospital inpatient stays, paying a fixed amount per MS-DRG rather than reimbursing each individual service.
- Upcoding, The practice of assigning a higher-severity MS-DRG, ICD-10 code, or CPT code than the patient's actual condition warrants, resulting in higher Medicare or insurer reimbursement.
- ICD-10 Code, The International Classification of Diseases, 10th Revision, code used to identify patient diagnoses on hospital claims, replacing ICD-9 in the U.S. on October 1, 2015.
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About This Definition
This definition is part of the HospitalCostData Hospital Pricing Glossary, 47 terms explaining hospital costs, quality ratings, and healthcare billing. Written for patients, journalists, researchers, and healthcare professionals.