Health Information Management

Q&A: Using case-mix index to track CDI efforts

CDI Strategies, February 28, 2013

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Q: Our case-mix index (CMI) seems to be dropping and management is putting pressure on the CDI staff to bring it back up, but I have heard that CDI programs should not be analyzed based on the (CMI) alone. Do you have any tips or advice about how I should track the CMI and how our efforts affect it?

A: The CMI is the sum of all your facility’s MS-DRG relative weights (RW), divided by the number (volume) of Medicare cases for the year. If you decide to review your CMI for short periods of time (e.g., per quarter), make sure you compare the quarter of interest to the previous year as seasonal variation in healthcare affects the CMI. You may also want to separate surgical cases from medical cases as the volume and type of surgical cases can impact surgical CMI. 
Do not compare CMI data month over month as far too much variability in data exists. A given specialty may have its annual convention every April leading to fewer expensive surgical cases, for example, or perhaps you work at a facility in Denver, where every winter sees an increase in orthopedic cases due to skiing accidents. A low CMI may denote MS-DRG assignments that do not adequately reflect the resources used to treat Medicare patients, but it is important to remember that the CMI is also affected by:
  • Types of services provided by the hospital
  • Volumes of medical and surgical cases
  • DRG assignments
  • Quality of documentation
  •  Changes in federal guidelines e.g., the reassignment of diagnoses as CCs/MCCs (such as when acute renal failure was reassigned from an MCC to a CC)
Because surgical MS-DRGs are higher weighted than medical MS-DRGs (because the cost of the surgery is typically higher and therefore represented in higher RW) an increase in the volume of surgical cases can increase the overall CMI. Conversely, when the volume of surgical cases is flat, and the CMI increases, such an increase represents an increasing complexity of medical patients seen by the facility.
Keep in mind economic factors as well. During the recent recession many people postponed elective surgeries and delayed healthcare, which negatively affected hospitals’ CMI, especially if they were considered a more elite or expensive healthcare provider in the community because consumers looked lower cost alternatives. 
When analyzing you CMI data, be sure to factor out extenuating circumstances. Look to assess data:
  • With and without tracheostomies
  • Seasonal shifts in diagnoses
  • Volume of symptom or low-weighted DRGs
  • Changes in surgical volumes (shifts of certain procedures to the outpatient setting)
  • Recovery Auditor targeted DRGs
  • Pairs or triplets of DRGs
Fluctuations in patient admissions and surgical staff/services make it difficult to use the CMI as an accurate measurement of CDI programs’ influence on overall facility finances. Believe it or not, a high volume of one day stays can negatively impact CMI as these short stay cases are often reimbursed under the lowest relative weight due to an absence of CCs and MCCs. As such, CDI is one of many departments that can impact CMI. That’s why I consider CMI an organizational metric rather than a departmental one.
When I managed a CDI department, I evaluated both the medical and surgical CC/MCC capture rate based on PEPPER data. The great thing about PEPPER data is that it is updated quarterly and allows one to see movement (by quarter) over a three-year time period. Therefore, you hope to see a favorable change as CDI is implemented and/or CDI targets an opportunity identified within the PEPPER data.  
You can also capture changes in severity of illness (SOI) and risk of mortality (ROM) if your organization is monitoring these values with the goal being an increase of cases with a value of major/3 and extreme/4 and a decrease in cases with moderate/2 and minor/1. 
Lastly, you can track the frequency with which a particular diagnosis occurs such as decreasing the volume of claims without CHF specificity. I strongly encourage CDI departments to focus on a few problem areas and monitor their progress in addressing these vulnerabilities. I prefer a targeted approach where one can focus CDI resources than a broader approach where quantity of reviews may be emphasized without a specific focus, which can negatively impact the quality of the reviews. 
It is particularly helpful for new CDI staff to have a specific focus to guide their review process allowing their successes to be measured. For example, the department may have a diagnosis of the month or quarter such as CHF. This approach does not preclude the identification of other incomplete, vague or missing diagnoses, but allows one to better capture the impact of CDI than only when a CC or MCC is added, as not all CDI efforts will result in a change in DRG assignment or movement in the CMI.
Either way, compare your data against a pre-CDI baseline and then again post-CDI implementation. Perform routine data analysis by specific time period (quarterly, for example), by medical and surgical service lines, as well as by physician specialties. Also, be patient: There is often a lag in data, so it can be six months to a year before the impact of CDI is evident in data metrics.
Editor’s Note: Cheryl Ericson, MS, RN, CCDS, CDIP, AHIMA Approved ICD-10-CM/PCS Trainer, CDI Education Director for HCPro Inc., answered this question which was originally published on the ACDIS Blog. Contact her at For information regarding CDI Boot Camps offered by HCPro visit

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