Case Mix Index (CMI) 

Case Mix Index (CMI) is the average relative value or weight assigned to a Medicare Severity-Diagnosis Related Group (MS-DRG) of patients in a medical care environment.  For example, a hospital's inpatient discharges.  The CMI reflects the diversity & clinical complexity, and determines the allocation of resources to care for and/or treat all patients in the clinical setting.  A higher CMI indicates a more complex and resource-intensive case load and, typically, yields a higher reimbursement rate.

CMS defines CMI as:  "the average MS-DRG relative weight calculated by summing the MS-DRG weights for all Medicare discharges and dividing by the number of discharges."

Patients are classified into an MS-DRG, which represents groups having the same condition (based on principal and secondary diagnoses, age, procedures performed, discharge status and gender), complexity (the presence of co-morbidity and/or complications) and needs.  In order to calculate CMI, weights are assigned to each MS-DRG by CMS.  These MS-DRG weights reflect the national "average hospital resource consumption" by patients for that particular MS-DRG, relative to the national "average hospital resource consumption" of all patients.  Although the MS-DRG weights are based on resource consumption by Medicare patients, it can be applied to all patient discharge data during the course of a calendar year, or specified timeframe.  The CMI is then calculated by averaging the MS-DRG weight of patients discharged with the calendar year or specified timeframe (i.e. the sum of MS-DRG weights divided by the number of patients).

In QualityAdvisor, CMI metrics are available on the following analyses for both CareScience Analytics and 3M™ APR DRG risk methodologies:

  • Custom Query (Facility only)
  • Custom Comparison (Facility only)
  • Facility Profiling

The CMI metrics on the Custom Query & Custom Comparison analyses are calculated at the patient level while the CMI metric on the Facility Profiling analysis is calculated at the facility level.

Note: Due to the inclusion and exclusion differences between the facility-level CMI and the patient-level CMI it is possible that the CMI FY value for a facility on the Facility Profiling analysis may not match the patient-level CMI on either the Custom Query or Custom Comparison analyses for the same facility.

  • The facility-level CMI FY value is calculated based on all inpatients (total cases) for the fiscal year, and includes both weighted and non-weighted MS-DRGs.
  • The patient-level CMI is calculated based on inpatients that have an MS-DRG that carries a weight assigned (i.e. MS-DRG 998 and 999 are not included.)

As a result, total cases used to calculate facility-level CMI FY may not match the total cases used to calculate patient-level CMI for the same facility and therefore could yield different CMI results.

Custom Query & Custom Comparison

The patient-level CMI metrics on the Custom Query and the Custom Comparison analyses allow you to report on any attribute in QualityAdvisor, including by practitioner. However, the patient level CMI metrics only include ICD-10 MS-DRGs that have been given a weight by CMS. The following two MS-DRGs are not assigned a weight, and are therefore excluded from the patient-level CMI calculation:

  • MS-DRG 998: Principal Diagnosis Invalid as Discharge Diagnosis
  • MS-DRG 999: Ungroupable

Since these two MS-DRGs are excluded from the calculation, it is possible that the total cases used to calculate CMI may not match the total cases for the report population.

  • The Case Mix Index - Total Cases metric on these two analyses show the number of cases used to calculate the CMI.

cmi attribute on the set-up analysis tab

The patient-level CMI metric is calculated by summing the MS-DRG weight for each discharge (based on the attribute selected within the reporting prompts) and dividing the total by the number of discharges for the selected population. If the reporting timeframe spans multiple CMS fiscal years, the MS-DRG weights applicable to the fiscal year in which the patients' discharge date occurred is considered.

CMI Examples with One Fiscal Year:

MS-DRG

Fiscal Year

# of Patients

Weight

001

FY17

50

.5

002

FY17

100

1.5

CMI = (50)(.5) + (100)(1.5) / 150

CMI Examples with Multiple Fiscal Years:

MS-DRG

Fiscal Year

# of Patients

Weight

001

FY17

50

.5

001

FY16

50

1.0

002

FY17

100

1.5

CMI = (50)(.5) + (50)(1.0) + (100)(1.5) / 200

Facility Profiling

The CMI metric on the Facility Profiling analysis is calculated at the facility level per fiscal year (FY), and is based on data in the Comparative Database.

The CMI FY value is calculated based on all inpatients (total cases) for the fiscal year regardless of MS-DRG weight assignment. Since the CMI FY includes both weighted and non-weighted MS-DRGs, the total cases used to calculate facility-level CMI FY may not match the total cases used to calculate patient-level CMI for the same facility.