What is the MSSP (Medicare Shared Savings Program)?

Definition

The Medicare Shared Savings Program (MSSP) is the largest Accountable Care Organization program in traditional Medicare, established by Section 3022 of the Affordable Care Act and operational since 2012. Participating ACOs — groups of physicians, hospitals, and other providers — accept accountability for the quality and total cost of care of a population of attributed Medicare fee-for-service beneficiaries, and share in savings (or, in some tracks, losses) against a financial benchmark CMS calculates annually.

ACO structure

An ACO in MSSP is a legally formed entity, typically a Tax Identification Number-bearing organization or a contractual joint venture, that aggregates participating Medicare-enrolled provider Taxpayer Identification Numbers under a single accountability structure. Participants commit to a multi-year agreement with CMS (currently five years), a defined participation track that determines the level of upside and downside financial risk, quality measurement and reporting using CMS’s specified ACO quality measure set, and participation in CMS data-sharing arrangements that provide claims data on attributed beneficiaries.

MSSP participants retain their existing Medicare billing relationships. ACO participation does not change how individual providers bill Medicare for services — the ACO accountability layer sits on top of underlying fee-for-service claims.

Beneficiary attribution

CMS attributes Medicare fee-for-service beneficiaries to MSSP ACOs based on where they receive the plurality of their primary-care services, measured by allowed charges for qualifying primary-care services. Attribution is retrospective for most tracks — beneficiaries are assigned at year-end based on their actual utilization pattern — though prospective attribution variants exist. Beneficiaries can also voluntarily align with an ACO, a mechanism CMS expanded to strengthen ACO–patient continuity.

Shared-savings and shared-loss arrangements

The financial mechanic of MSSP is comparison of total Medicare Part A and Part B spending for attributed beneficiaries against a benchmark CMS calculates. The benchmark incorporates historical spending for the attributed population, trended forward, with adjustments for regional spending, risk-score changes, and prior performance.

• Basic Track levels A and B — One-sided arrangements with upside-only shared savings. Limited to new ACOs and restricted-duration participation.
• Basic Track levels C, D, E — Increasing levels of two-sided risk, with shared savings rates rising as downside risk increases.
• Enhanced Track — Highest shared-savings rate paired with highest downside-risk exposure. Comparable to the legacy Track 3 structure.

The redesigned program structure pushes ACOs onto an on-ramp from one-sided to two-sided risk over their participation agreement, consistent with the broader Medicare policy shift toward two-sided accountability.

Quality reporting

MSSP ACOs report on the CMS-specified ACO quality measure set, which includes measures of patient experience, care coordination, preventive health, and at-risk population management. Performance on the quality measure set affects the percentage of available shared savings the ACO is eligible to receive — strong quality performance preserves shared-savings eligibility, weak performance reduces it. The measure set is harmonized with HEDIS and CMS Web Interface measures.

Connection to APCM

MSSP ACOs frequently adopt APCM at network scale. The fit is direct: APCM reimburses the care-coordination, panel-management, and chronic-disease-management activities that an ACO needs its participating primary-care practices to perform in order to manage total cost of care and produce shared savings. Because APCM is a Physician Fee Schedule code billed by the practice under its own TIN — not a separately negotiated ACO arrangement — practices in an MSSP ACO can adopt APCM without disturbing ACO participation, and the ACO can encourage adoption as a network-level strategy.

For practices billing APCM with behavioral health add-on codes (G0568-G0570) under the same TIN, the behavioral health activity also contributes to managing the cost of behavioral-comorbid chronic conditions — depression, anxiety, and substance use disorder are well-documented drivers of total medical cost. In partnership-based BHI implementations, the behavioral health partner is paid a fixed Fair Market Value Management Services Agreement fee, not a share of shared savings, preserving CMS anti-kickback and Stark Law alignment.

Practical positioning

For a primary-care practice evaluating APCM in the context of MSSP participation, the central considerations are:

• APCM is an additive Physician Fee Schedule revenue stream, not a competing financial arrangement with ACO participation.
• The care-management capability APCM requires is largely the same capability the ACO needs the practice to deploy for total-cost-of-care management.
• Adoption sequencing — whether to add APCM before, alongside, or after deeper ACO risk-track movement — is a practical question that depends on the practice’s current care-management infrastructure.

Primary sources

• Centers for Medicare & Medicaid Services. Medicare Shared Savings Program: Final Rule. 42 CFR Part 425.
• Centers for Medicare & Medicaid Services. CY2023 Medicare Physician Fee Schedule Final Rule — MSSP redesign provisions.
• National Association of ACOs (NAACOS) policy briefings on Basic and Enhanced Track structure.
• MedPAC reports on Medicare ACO performance.

Related concepts

For the full-risk ACO model, see What is ACO REACH?. For the broader payment shift, see What is value-based care?. For the Physician Fee Schedule program that drives MIPS performance, see What is the Quality Payment Program?. For the quality measurement framework, see What is HEDIS?. For APCM itself, see What is APCM? and The APCM Opportunity.