What Is Case Mix Index?
Case mix index, often abbreviated CMI, is the average relative weight of all DRGs assigned to a hospital's inpatient discharges over a specific period, providing a single summary measure of overall patient acuity and resource intensity. RHIA candidates should understand CMI as a key indicator watched closely by finance and HIM leadership because it directly influences reimbursement levels.
Calculating Case Mix Index
Case mix index is calculated by summing the relative weights of all DRGs for discharges in a given period and dividing by the total number of discharges. A higher CMI indicates that, on average, the hospital treated more complex and resource-intensive cases, which corresponds to higher average reimbursement per case under a DRG-based payment system.
Factors That Influence Case Mix Index
- The actual clinical complexity and acuity of the patient population served by the hospital
- The completeness and specificity of clinical documentation, since incomplete documentation can understate true DRG weight
- Coding accuracy and the coder's ability to correctly apply coding guidelines to capture all relevant diagnoses and procedures
- Changes in service lines, such as adding a transplant program that shifts the hospital toward higher-weighted DRGs
Case Mix Index and Clinical Documentation Improvement
Because CMI is sensitive to documentation completeness, many hospitals track CMI as a key performance indicator for their clinical documentation improvement programs, monitoring for increases that reflect more accurate capture of clinical complexity rather than any change in actual patient acuity, which would represent legitimate improvement rather than improper upcoding.
Distinguishing Legitimate CMI Improvement from Upcoding
A legitimate increase in CMI driven by improved documentation accuracy reflects diagnoses and complications that were always clinically present but previously undocumented or uncoded. Upcoding, by contrast, involves assigning codes not supported by clinical documentation, which is a compliance violation subject to significant penalties under federal fraud and abuse laws.
Using Case Mix Index for Benchmarking
Hospitals compare their CMI over time and against similar facilities to identify trends, evaluate the financial effect of service line changes, and assess whether coding and documentation practices are keeping pace with actual clinical complexity.
Exam Tip
Be able to calculate a simple case mix index given a set of DRG relative weights and discharge counts, and explain the distinction between legitimate documentation improvement and improper upcoding.