What Is the MS-DRG System?
The Medicare Severity Diagnosis Related Group system, known as MS-DRG, is a patient classification system used by the Centers for Medicare and Medicaid Services to determine inpatient hospital reimbursement under the Inpatient Prospective Payment System. Each inpatient stay is assigned to a single MS-DRG based on the principal diagnosis, secondary diagnoses, procedures performed, and other patient factors.
Major Diagnostic Categories
MS-DRGs are organized under 25 Major Diagnostic Categories, commonly called MDCs, which generally correspond to a single body system or condition category, such as diseases of the circulatory system or diseases of the digestive system. Every MS-DRG falls under one MDC.
How MS-DRGs Are Assigned
DRG assignment begins with grouping software analyzing the principal diagnosis to determine the appropriate MDC. The software then considers whether a significant procedure was performed, which may classify the case as surgical or medical within that MDC. Secondary diagnoses are then evaluated to determine whether they qualify as complications or comorbidities.
Complications and Comorbidities
Secondary diagnoses can affect DRG assignment through two designations:
- CC (Complication or Comorbidity): a condition that increases resource use and length of stay to a moderate degree
- MCC (Major Complication or Comorbidity): a condition that significantly increases resource use and length of stay
Many MS-DRGs exist in a three-tier structure representing cases without CC or MCC, with CC, and with MCC, reflecting escalating severity and resource use.
Relative Weights
Each MS-DRG is assigned a relative weight that reflects the average relative cost of treating patients in that group compared to the average Medicare patient. Higher relative weights indicate higher expected resource consumption and result in higher payment.
Payment Calculation
The base payment for an MS-DRG is calculated by multiplying the relative weight by the hospital's specific base payment rate, which accounts for factors such as wage index, teaching status, and disproportionate share status. Outlier payments may be added for unusually costly cases that far exceed the expected cost for a given DRG.
Importance of Accurate Coding
Because DRG assignment depends directly on coded diagnoses and procedures, coding accuracy and documentation specificity have a direct financial impact on hospital reimbursement. Missing a documented MCC, for example, could significantly reduce payment for an encounter.
The HIM Role
HIM coding professionals must ensure that principal diagnosis selection, secondary diagnosis capture, and procedure coding accurately reflect the clinical picture to support appropriate DRG assignment and compliant reimbursement.
Conclusion
The MS-DRG system links clinical documentation and coding directly to inpatient hospital reimbursement, making coding accuracy and severity capture essential components of revenue cycle performance.