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Case Mix Index: Beyond the Physician's Pen

Published on 

Wednesday, April 6, 2016

When I was first introduced to the concept of Case Mix Index (CMI) in the late 90’s, documentation in the medical record was handwritten. And I can remember understanding that CMI depends on the physician’s pen.

Flash forward to 2016 and the electronic health record. It is now fair to say that it all begins with the click of a button. I am not sure if it is due to the fact that I wrote my college term papers on a Brother Typewriter or from auditing electronic records remotely, the “click of the button,” has not improved the telling of the patient’s story which is at the heart of what needs to happen.

In fact, it seems to me that it is harder than ever to find proof that your “patients are sicker.” Understanding CMI is a good way to answer the question of “how do I know my patients are sicker.” However, to understand CMI you need to first understand the basic fundamentals of the Inpatient Prospective Payment System (IPPS) and how a Coder in a hospital determines the Diagnosis-Related Group (DRG) assignment for every hospital inpatient stay.


In 1983, Congress mandated the Inpatient Prospective Payment System (IPPS) for all Medicare inpatients. IPPS uses Diagnosis-Related Groups (DRGs) to determine reimbursement for hospitals.  

Beginning October 1, 2007 the DRG system began transitioning to a new system called Medicare Severity MS-DRG. The transition to MS-DRGs allowed for an improved accounting of a hospital’s resource consumption for a patient and the patient’s severity of illness.  

Assigning a DRG:

Principal Diagnosis:

The Uniform Hospital Discharge Data Set (UHDDS) defines the Principal Diagnosis as “the condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care.”

Comorbidities and Complications (CCs and MCCs):

These are conditions that increase a patient’s resource consumption and may cause an increase in length of stay compared to a patient admitted for the same condition without a co-morbidity or complication.  When the DRG system transitioned to MS-DRGs the comorbidities and complications were divided into three levels. The three levels are DRGs without a CC or MCC, DRGs with a CC and DRGs with a MCC.

  • Comorbidities (CC) are the conditions that patients “bring with them” when they are admitted to a hospital and continue to require some type of treatment or monitoring while in the inpatient setting.
  • For example: A patient with a history of atrial fibrillation is continued on his home medications and placed on telemetry monitoring.
  • A patient with a history of Diabetes is placed on pattern blood sugars with sliding scale insulin
  • A patient has a history of hypercholesterolemia and is continued on their home Statin therapy.
  • Complications (CC) are those conditions that occur during the inpatient hospitalization.
    For example:
  • A patient undergoes hip surgery and experiences acute post-op blood loss anemia in the peri-operative period requiring serial Hemoglobin and Hematocrit checks and possibly blood transfusions. 
  • Major Comorbidities and Complications (MCCs): DRGs with MCCs reflect the highest level of severity. For example:
  • A patient with chronic systolic heart failure is admitted for a GI bleed, becomes volume overloaded and develops acute on chronic systolic heart failure during the admission.
  • A patient with a history of chronic obstructive pulmonary disease undergoes surgery and develops post-op respiratory failure.

As many times as we have heard it said it remains true, if you don’t document it then it wasn’t done or in the case of DRG assignment it wasn’t present and treated during the hospitalization. A Coder’s ability to code to the most appropriate DRG is dependent upon the Physician documentation in the medical record.   Coding Guidelines do not allow coders to interpret lab findings, radiology findings, EKGs or pathology reports to assign diagnosis codes.

A successful DRG program in a hospital is dependent on the Physician providing a complete accounting of a patient’s Principal Diagnosis, comorbidities and complications, any procedures performed, the plan of care and the patient’s discharge status.  


A patient presents with chest pain and has a known history of GERD. A Myocardial Infarction (MI) was ruled out based on EKG and Cardiac Enzymes and the patient was discharged home with a new prescription for Prilosec. In this case chest pain is a symptom code and a more specific diagnosis would be chest pain related to GERD. However, if the only diagnosis written by the Physician in the record is chest pain then the coder can only assign the code for unspecified chest pain.

This is why Coders and in more recent years Clinical Documentation Specialist send queries to Physicians. As far back as 2007, CMS has indicated that “we do not believe there is anything inappropriate, unethical or otherwise wrong with hospitals taking full advantage of coding opportunities to maximize Medicare payment that is supported by documentation in the medical record.” (Source: Federal Register / Vol. 72, No. 162 / Wednesday, August 22, 2007 / Rules and Regulations – page 47180)

Diagnosis-Related Group (DRG) is a diagnosis classification that groups patients that have a similar resource consumption and length-of-stay.

Relative Weight (RW) is a numeric weight assigned to each DRG that is indicative of the relative resource consumption associated with that DRG.

Case Mix Index (CMI) is defined by CMS as representing “the average diagnosis-related group (DRG) relative weight for the hospital. It is calculated by summing the DRG weights for all Medicare discharges and dividing by the number of discharges.”

GPA Example:

(A=4 grade points / B=3 grade points / C=2 grade points / D = 1 grade point / F = 0 grade points)

Example Student Transcript


Credit Hours


Grade Points





Chemistry Lab




English 101








Sum of Credit Hours Attempted: 10

30 Total Grade Points

Formula for GPA: Total Grade Points ÷ Sum of Credit Hours = GPA

30 ÷ 10 = 3.0 GPA

Case Mix Index Example A:

DRGs Coded
DRGDRG DescriptionRelative Weight
193Simple Pneumonia and Pleurisy with MCC1.4261
194Simple Pneumonia and Pleurisy with CC0.9695
195Simple Pneumonia and Pleurisy without CC/MCC0.7111
313Chest Pain0.6621
4 Total DRGs codedSum of Relative Weights: 3.7688

Formula for Case Mix Index: Sum of Relative Weights ÷ Total Number of DRGs Coded = CMIExample A CMI: 3.7688 ÷ 4 = 0.9422 Case Mix IndexExample B: The Potential Impact Physician Queries can have on DRG Assignment

DRGs Coded
Pre-Query DRGQuery OpportunityPost-Query DRGNew Relative Weights
193Query clarified patient had aspiration pneumonia1771.9033
194No Query Opportunity1940.9695
195Home medications included Lasix, Lisinopril & Digoxin. Echocardiogram within past 6 months showed Ejection Fraction 30%. Query clarified patient has chronic systolic heart failure1040.9695
313Cardiac cause of chest pain ruled out. Query clarified chest pain due to GERD3920.7400
4 Total DRGs codedSum of Relative Weights: 4.5823

Example B CMI: 4.5823 ÷ 4 = 1.1456 Case Mix Index

“The higher the case mix index, the more complex the patient population and the higher the required level of resources utilized. Since severity is such an essential component of MS-DRG assignment and case mix index calculation, documentation and code assignment to the highest degree of accuracy and specificity is of utmost importance.” (Source: Optum 360 2016 DRG Expert)

Challenges for Hospitals:

Understanding what can make your hospitals CMI fluctuate?

  • A decrease in CMI may be reflective of:
  • Non-specific documentation by the Physician
  • Increase in Medical Volume with a decrease in Surgical Volume as Surgical DRGs have a higher Relative Weight.
  • Surgeons being on vacation
  • Physicians being unresponsive to Coder and Clinical Documentation Specialist queries
  • An increase in CMI may be reflective of:
  • Tracheostomy procedures that have an extremely high Relative Weight
  • Ventilator patients
  • Open Heart Procedures
  • Improved Physician Documentation
  • Improved Physician response rate to queries resulting in an improved CC / MCC capture rate

Realizing the Importance of every Medical Professional’s role in the success of a hospital’s DRG program:

  • The Physician’s Role: Is to provide complete and accurate documentation of a patient’s Principal Diagnosis, comorbidities and complications, any procedures performed, the plan of care and the patient’s discharge status in the medical record.
  • The Clinical Documentation Specialist’s Role: Is to perform concurrent medical record reviews and ask queries whether verbal or written when indicated.  
  • The Coder’s Role: May be concurrent medical record review or a retrospective review after discharge; also ask queries when indicated.

In 2013, the American Health Information Management Association (AHIMA) published the practice brief Guidelines for Achieving a Compliant Query Practice. The AHIMA brief states that “A query is a communication tool used to clarify documentation in the health record for accurate code assignment. The desired outcome from a query is an update of a health record to better reflect a practitioner’s intent and clinical thought processes, documented in a manner that supports accurate code assignment.”


We are now six months post ICD-10-CM/PCS implementation. Has this transition impacted CMI? To answer this question I analyzed paid claims data from our sister company RealTime Medicare Data (RTMD). The following tables compare CMI data from October through December of 2014 compared to 2015.

Figure 1: Alabama CMI Compare Pre and Post ICD-10-CM/PCS Implementation

Figure 2: South Carolina CMI Compare Pre and Post ICD-10-CM/PCS Implementation

Figure 3: Texas CMI Compare Pre and Post ICD-10-CM/PCS Implementation

So far, it appears that the transition has not had a negative impact on CMI but it is still early and MMP, Inc. will continue to keep an eye on the trends and report key findings to our readers. In the meantime, remember that a successful DRG program is dependent on accurate documentation. Addressing issues that can impact CMI will enable you to capture the most accurate severity of illness, have a positive impact on reimbursement and support the medical necessity of inpatient admissions.

Article Author: Beth Cobb, RN, BSN, ACM, CCDS
Beth Cobb, RN, BSN, ACM, CCDS, is the Manager of Clinical Analytics at Medical Management Plus, Inc. Beth has over twenty-five years of experience in healthcare including eleven years in Case Management at a large multi-facility health system. In her current position, Beth is a principle writer for MMP’s Wednesday@One weekly e-newsletter, an active member of our HIPAA Compliance Committee, MMP’s Education Department Program Director and co-developer of MMP’s proprietary Compliance Protection Assessment Tool.

This material was compiled to share information.  MMP, Inc. is not offering legal advice. Every reasonable effort has been taken to ensure the information is accurate and useful.