What is case mix index (CMI)?

Case mix index is a measure used by the Centers for Medicare and Medicaid Services (CMS) to determine hospital reimbursement rates for Medicare and Medicaid beneficiaries. This measure reflects the diversity, complexity, and severity of patient illnesses treated at a given hospital or other healthcare facility.

Higher case mix index values indicate that a hospital has treated a greater number of complex, resource-intensive patients, and the hospital will be reimbursed at a higher rate.

Case mix index is calculated by adding up the relative Medicare Severity Diagnosis Related Group (MS-DRG) weight for each discharge, and dividing that by the total number of Medicare and Medicaid discharges in a given month and year.

Why is case mix index an important healthcare metric?

Not only does case mix index play a central role in hospital finances, it is also an important indicator of hospital performance and clinical documentation.

Two hospitals with comparable patient populations and surgical capacity could, for instance, report two different case mix index values. Though the patients that they care for and the services that they provide are largely the same, a lower case mix index could indicate that one hospital is documenting its cases less effectively.

It’s important to remember, however, that this metric can be impacted by many different factors, including:

  • Coding accuracy
  • Documentation specificity
  • High volumes of highly weighted DRGs (like organ transplantations, cardiothoracic surgeries, or neurosurgeries)
  • Annual updates to relative MS-DRG weights