Q&A: Understanding case mix index

Q: I’ve heard that an increase in the volume of surgical cases can increase a facility’s overall case mix index (CMI) and that when the volume of surgical cases is flat, movement in the CMI represents increasing complexity of patients cared for in that facility. Could you explain these concepts a bit more for me? 

A: The CMI is a very popular measure used to evaluate CDI effectiveness. Unfortunately, it is not actually the best measure due to the number of factors that influence a facility’s CMI.

I used to joke that I wasn’t going to take credit when the CMI soared because I would also have to take credit when it tanked. Program managers and CDI staff need to understand the patient population, the types of procedures performed, and the specialties cared for at your facility all affect the CMI.

So, how does an increase in the volume of surgical cases increase the overall CMI? The CMI is the average of relative weights of DRGs assigned to a specific population or group. For example, to compute the CMI for all patients discharged in the month of March, take the average of the relative weights for every DRG assigned for discharges within that time period. A relative weight reflects the resource consumption associated with each MS-DRG. In general, the relative weights assigned to surgical DRGs are higher to reflect the increased resources required for a surgical procedure and the hospital stay. If the volume of surgical cases drops for any reason, for example two surgeons take vacations in the same month, the total CMI for the organization will likely decrease due to the fact there are less admissions with higher relative weights.

Now, let’s consider the case of a CMI increasing when the volume of surgical cases is flat. If the surgical volume remains constant, any movement in the organizational CMI can be explained by variations in the medical patients admitted during that timeframe. For this reason, it’s helpful to compute the CMI for the medical population separately from the surgical. This allows you to look at each population separately.

Again, it’s important to understand that many issues can affect CMI. For hospitals located in areas in which there is a seasonal influx of patients, there may be a corresponding seasonal effect to the CMI. For example, in the winter months, many patients of Medicare age relocate to warmer climates such as Florida or Arizona. This influx of “snowbirds” likely will lend to an increase in CMI.

Gatekeeping efforts also affect the CMI. If patients are admitted with principal diagnoses that likely don’t require an inpatient stay (symptoms, transient ischemic strokes, syncope, etc.), the relative weights will reflect the lower resource use and the overall CMI will be lower.

All that said, CDI efforts can have an effect on the CMI by ensuring documentation supports each patient’s complexities and resource use. But, it’s important to understand the effect of the other factors, too. Metrics such as the CMI need to always be considered within the context of your organization’s population and the care provided.

Here are a few more articles you might find helpful regarding CDI metrics:

• “Note from the Instructor: CDI leadership and KPIs: Long live metrics”
• “Part of the picture: Measuring more than CMI”
• “Physician Advisor’s Corner: Your CMI dropped: What is (and isn’t) in your control”

Editor’s Note: Laurie L. Prescott, RN, MSN, CCDS, CDIP, CRC, CDI education director at HCPro in Middleton, Massachusetts, answered this question. Contact her at lprescott@hcpro.com. For information regarding CDI Boot Camps visit www.hcprobootcamps.com/courses/10040/overview.