What Is Case Mix Index
Case Mix Index, or CMI, is a numeric value representing the average relative weight of all cases treated by a hospital or specific service line over a defined period. Because DRG relative weights reflect resource intensity, CMI serves as a proxy measure for the overall clinical complexity and acuity of a hospital's patient population.
Calculating CMI
CMI is calculated by summing the relative weights of all discharges within the period and dividing by the total number of discharges. A hospital treating a higher proportion of complex, resource-intensive cases will have a higher CMI than one treating predominantly routine cases, assuming coding accurately captures the clinical picture.
Impact on Reimbursement
Because Medicare inpatient reimbursement is calculated by multiplying the DRG relative weight by the hospital's base payment rate, CMI directly influences overall inpatient revenue. A higher CMI generally results in higher average reimbursement per case, reflecting the greater resources required to treat more complex patients. However, CMI must accurately reflect true clinical acuity, since artificially inflating CMI through improper coding constitutes fraud.
Documentation Improvement and CMI
Clinical Documentation Improvement, or CDI, programs play a central role in ensuring CMI accurately reflects patient acuity. CDI specialists review documentation concurrently with the patient stay, querying physicians when documentation is ambiguous, incomplete, or fails to capture conditions that affect DRG assignment, such as complications, comorbidities, or major complications and comorbidities.
Common CDI Focus Areas Affecting CMI
- Capturing all clinically significant secondary diagnoses
- Clarifying acuity terms such as sepsis, acute respiratory failure, or malnutrition
- Ensuring present on admission indicators are accurate
- Verifying principal diagnosis selection reflects the primary reason for admission
Monitoring Strategies
Organizations track CMI trends over time, comparing month-to-month and year-over-year changes, and investigate significant unexpected shifts. CMI should also be benchmarked against similar facilities using data from sources such as the Medicare Provider Analysis and Review file to identify whether an organization's case mix aligns with expected patterns for its patient population and service offerings.
Ethical Considerations
While CDI programs legitimately improve documentation specificity to accurately reflect care provided, any effort to inflate CMI without corresponding clinical support crosses into improper upcoding, exposing the organization to compliance risk and potential False Claims Act liability.