MS-DRG Assignment, Sequencing Rules, and Reimbursement Methodologies
How a claim becomes a DRG
The grouper assigns a Major Diagnostic Category based on the principal diagnosis, with exceptions for MDC 14 (pregnancy) and MDC 15 (newborns). Within the MDC, a qualifying operating-room procedure drives a surgical DRG; without one, the principal diagnosis drives a medical DRG. Only procedures on the MDC's OR list are surgical. The principal procedure is the one most closely related to the principal diagnosis - if it is not an OR procedure, it does not create a surgical DRG.
CCs and MCCs
Secondary diagnoses that are complications/comorbidities raise severity, but only if they are present on admission, are not the principal diagnosis, and do not appear on the CC/MCC exclusion list for that principal diagnosis. That exclusion list is the single most common reason a severity level 'disappears' on review.
Reimbursement math
Medicare IPPS pays per discharge: DRG weight multiplied by the hospital base rate, adjusted for wage index, IME, DSH/uncompensated care, and for cost outliers or transfer status. Two hospitals with identical DRGs receive different dollars. Know the HAC Reduction Program, HRRP readmissions, and DRG mismatches as payment risk areas.
Sequencing rules
Principal diagnosis first, then conditions that meet UHDDS criteria, then the principal procedure as the first-listed procedure. Sequencing that ignores UHDDS definitions can produce a clinically plausible but non-compliant DRG.
Worked example
Principal: COPD with acute exacerbation (J44.1). Secondary: acute systolic heart failure (I50.21), POA = Y. The heart failure is a CC and raises severity - unless the CC exclusion list for J44.1 excludes it. An unspecified I50.9 often fails to qualify while I50.21 succeeds. Specificity is money.