Q&A: Surgical cases and CMI

Q: I’ve heard that when the volume of surgical cases is flat, any movement in the case mix index (CMI) represents an increased complexity of the patients. Can you explain what that means?

A: When a facility does the same amount of the same procedures on the same type of patient population their surgical CMI will remain the same or become “flat.” To increase your CMI or show movement in the CMI, the complexity of the patient population will need an increase in the complexity of the patient population being admitted could explain an increase in your CMI.

To calculate the CMI, we would use the coder assigned final MS-DRG relative weight (RW) for each encounter within your data set (be it a time period, service line, etc.). We'd then take the RW value and multiply it by the number of patients discharged with that particular medical or surgical MS-DRG. Once this is completed, we’d add up all the values and divide by the number of patients used in your calculations. This will give you the CMI for whatever timeframe you are using, whether it is a month, a quarterly report, or even a yearly report. The CMI can also be calculated for a single provider, group of providers, or surgical cases versus medical cases.

More than one thing typically influences the CMI at a time and many departments (not just CDI) contribute to an increase in the CMI. Some reasons for an increase could be related to the addition of a different type of surgeon or a different type of surgery being performed at the facility. The case management or utilization review department can also make an impact on the CMI because it is influenced by the status of the patients (e.g., outpatient, observation, inpatient). For example, inpatient admissions that should have been in observation status would affect your CMI because these patients are usually admitted with a principle diagnosis that leads to a MS-DRG with a lower RW.

CMI usually varies by the time of year as well. For example, wintertime often sees an increase in the volume of patients because of the weather. Implementing a CDI department could also increase the CMI because their reviews often result in the capture of conditions that are classified as CC/MCCs, which could lead to the final MS-DRG being a MS-DRG with a higher RW.

You are correct that an increase in patient complexity (i.e., patients with a higher severity of illness/risk of mortality) can also result in a higher CMI. This is because those additional secondary diagnosis CC/MCCs (indicating the patient’s complexity) may lead to a higher RW.

One last thing: you should always compare  your calculated CMI to the same timeframe the previous year. If you are looking for either an increase or decrease in the month of November 2021, you would compare it to November 2020, not to the months of October 2021 and December 2021. To measure the difference of implementing a CDI department, you would want to compare it to the same month in the previous year prior to the implementation.

Editor’s Note: Sharme Brodie, RN, CCDS, CCDS-O, CDI education specialist and CDI Boot Camp instructor for HCPro in Middleton, Massachusetts, answered this question. For information, contact her at sbrodie@hcpro.com. For information regarding CDI Boot Camps, click here. This article originally ran in June 2019 and has been updated according to all new coding and documentation guidelines.