Clinical documentation integrity (CDI) specialists and inpatient coders are well-versed in how the Inpatient Prospective Payment System (IPPS) affects hospital reimbursement. But many are less familiar with why Medicare Severity Diagnosis Related Group (MS-DRG) relative weights change each year—or that the formula used to calculate them is about to shift. The transition is expected to temporarily lower case mix indexes (CMIs) as MS-DRGs are recalibrated under the new approach.
MS-DRGs are the foundation of payment under the IPPS, which governs Medicare Part A hospital reimbursement nationwide except in Maryland. Each MS-DRG weight reflects the relative cost of treating patients in that category compared with the average cost of treating all inpatient cases, according to the Centers for Medicare & Medicaid Services (CMS).
Federal law requires CMS to update MS-DRG classifications and relative weights annually to reflect changes in clinical practice, technology, and resource use. One of the biggest drivers of these adjustments is declining lengths of stay (LOS). Every MS-DRG includes a geometric mean length of stay (GMLOS), a statistical measure of expected inpatient days. Medicare counts LOS from midnight to midnight and excludes the discharge day. Because room-and-board— including nursing and ancillary services—constitutes the most expensive hospital cost, shorter stays reduce resource use and ultimately reimbursement.
CMS is now preparing to adopt a market-based methodology for calculating MS-DRG weights. First introduced in the fiscal year (FY) 2021 IPPS Final Rule but later withdrawn, the approach has been revived with a new requirement: beginning in FY 2026, hospitals must report median payer-specific negotiated charges for Medicare Advantage (MA) plans by MS-DRG. These figures will gradually replace chargemaster prices in determining IPPS MS-DRG updates.
CMS plans to use this method to calculate MS-DRG weights for FY 2029. The agency expects only modest changes at first but says the long-term goal is to better align MS-DRG weights with actual inpatient resource use and true market value.
More than half of Medicare beneficiaries now enroll in MA plans. CMS pays MA organizations a prospective per-member rate based on expected healthcare needs, and MA plans then reimburse hospitals—typically using MS-DRGs or per diem rates—when members receive inpatient care.
One major reason for the methodological shift is CMS’s desire to reduce reliance on hospital chargemasters. Research shows that hospitals often inflate chargemaster prices to improve margins. High chargemaster rates serve several purposes: they set baseline charges for uninsured patients (many later written off), shape out-of-network billing, influence negotiations with private insurers, and increase the apparent value of bad-debt write-offs.
CMS argues that MA-negotiated charges are a more accurate reflection of the resources hospitals actually use. Although MA and fee-for-service (FFS) populations should theoretically have similar inpatient needs, hospitals contend that MA enrollees tend to be healthier and require fewer resources. MA plans also face incentives to control costs and have been shown to pay about 5.6 percent less for inpatient services than FFS Medicare, partly due to steeper discounts for short-stay admissions.
Despite these differences, CMS maintains that MA pricing more closely represents the market cost of inpatient care while still allowing hospitals to remain financially viable.
Related topic：
