The ICD-10 MS-DRG Definitions Manual contains the detailed grouper logic CMS uses for MS-DRG assignment. CMS states that the FY 2026 Definitions Manual files provide the complete documentation of ICD-10 MS-DRG grouper logic.
The manual organizes MS-DRGs by Major Diagnostic Category and includes the principal diagnoses, procedures, MCC/CC structure, exclusions, and other logic needed to understand how cases group.
As of September 2026, CMS identifies FY 2026 Version 43.1 as effective for discharges from April 1 through September 30, 2026. Version 44 begins October 1, 2026 for FY 2027.
The Physician Fee Schedule governs professional services and does not define inpatient MS-DRGs. HCAHPS concerns patient-experience measurement, while the National Correct Coding Initiative focuses primarily on coding edits designed to prevent inappropriate code combinations in other reimbursement contexts.
CDI specialists do not need to memorize every grouper pathway, but they should understand how principal diagnosis, secondary conditions, procedures, and CC/MCC classifications interact to determine the working and final MS-DRG.
CCDS Reference Topics: MS-DRG Definitions Manual; grouper logic; IPPS; current fiscal-year resources.
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