Accountable Care Organizations and Value-Based Care

Accountable Care Organizations (ACOs) represent a structural response to the fragmented, fee-for-service payment model that dominated US healthcare for decades, creating financial incentives that rewarded volume over outcomes. This page covers the regulatory framework governing ACOs, the mechanics of shared savings and risk arrangements, the classification boundaries between ACO program types, and the documented tensions that arise when aligning financial incentives across provider networks. Understanding ACO structure is foundational to interpreting how coordinated and integrated care models operate within the broader Medicare and commercial insurance landscape.

• Definition and scope
• Core mechanics or structure
• Causal relationships or drivers
• Classification boundaries
• Tradeoffs and tensions
• Common misconceptions
• Checklist or steps (non-advisory)
• Reference table or matrix

Definition and scope

An Accountable Care Organization is a network of physicians, hospitals, and other health care providers that voluntarily agrees to coordinate care for a defined patient population, accepting collective accountability for the cost and quality of that care. The Centers for Medicare & Medicaid Services (CMS) defines ACOs within the Medicare program under Section 3022 of the Affordable Care Act (ACA), codified at 42 U.S.C. § 1395jjj, which established the Medicare Shared Savings Program (MSSP).

The scope of ACO participation extends well beyond Medicare. Commercial insurers and state Medicaid programs operate parallel ACO frameworks, though the regulatory architecture differs across payers. As of the CMS program data published in 2023, more than 480 ACOs participate in the MSSP, collectively serving over 10.8 million assigned Medicare beneficiaries (CMS MSSP Fast Facts, 2023).

The value-based care umbrella is broader than ACOs alone. It encompasses bundled payment arrangements, patient-centered medical homes (PCMHs), and pay-for-performance programs. ACOs represent the most organizationally complex tier of value-based care because they require attribution of a patient population, multi-year financial risk arrangements, and reporting across a standardized quality measure set.

Core mechanics or structure

ACOs operate through a combination of patient attribution, quality measurement, and shared savings or loss calculations. The foundational mechanism is the benchmark expenditure: CMS or a commercial payer establishes a projected spending target for the ACO's attributed population, based on historical claims data. If the ACO's actual per-capita expenditures fall below the benchmark, a portion of the difference is returned to the ACO as shared savings. If spending exceeds the benchmark, the ACO may owe a shared loss payment, depending on the risk track elected.

Patient attribution does not require patient enrollment or opt-in. Under MSSP rules, Medicare beneficiaries are retrospectively attributed to an ACO based on which participating primary care provider they visited most frequently during the performance year. This mechanism means that patients may not be aware of their ACO assignment, a design choice with implications discussed in the misconceptions section.

Quality measurement under the MSSP is governed by a set of CMS-specified measures drawn from domains including preventive care, at-risk populations, patient experience (via Consumer Assessment of Healthcare Providers and Systems, CAHPS), and utilization. The 2023 ACO REACH model, which replaced the Global and Professional Direct Contracting models, uses a streamlined set of quality measures tied to health equity benchmarks (CMS Innovation Center, ACO REACH Model).

Risk tracks under MSSP include:
- BASIC Track (Levels A–E): A glide path from one-sided shared savings only (Level A) to increasing shared risk as the ACO progresses through levels.
- ENHANCED Track: Two-sided risk from inception, with higher shared savings rates (up to 75%) in exchange for bearing potential shared losses capped at 15% of the benchmark.

Causal relationships or drivers

The policy logic underlying ACOs traces directly to documented failures of fee-for-service (FFS) payment: FFS pays providers for each discrete service delivered, creating no incentive to prevent hospitalizations, eliminate duplicative testing, or coordinate post-acute transitions. The Medicare Payment Advisory Commission (MedPAC) has documented sustained patterns of preventable readmissions and duplicative imaging that FFS structures do not penalize (MedPAC Report to Congress, March 2023).

ACO adoption accelerated after the ACA's passage in 2010 for two interconnected reasons. First, the MSSP created a federal infrastructure for shared savings contracts that gave provider organizations a direct financial stake in population health outcomes. Second, CMS's Center for Medicare and Medicaid Innovation (CMMI), also created by the ACA, began testing alternative payment models—including Pioneer ACOs and Next Generation ACOs—that introduced progressively higher risk levels to evaluate whether financial accountability changes clinical behavior.

The demonstrated driver of ACO savings is reductions in acute care utilization: lower inpatient admissions, reduced emergency department visits, and more appropriate transitions to home health care services and rehabilitation and physical therapy services. A 2019 analysis published by CMMI found that MSSP ACOs generated $1.66 billion in net Medicare savings over 2013–2015, with physician-led ACOs outperforming hospital-led ACOs on savings rates (CMS MSSP Results, 2019).

Classification boundaries

ACOs are classified along three principal axes: payer type, risk arrangement, and organizational structure.

By payer type:
- Medicare ACOs operate under MSSP or CMMI models and are governed by CMS regulations at 42 C.F.R. Part 425.
- Medicaid ACOs are state-administered; Oregon's Coordinated Care Organizations and Massachusetts's Accountable Care Organizations represent two distinct state-level architectures with differing risk structures.
- Commercial ACOs are contractual arrangements between provider networks and private insurers; they lack a uniform federal regulatory framework.

By risk arrangement:
- One-sided (upside-only): ACOs share in savings but bear no financial loss exposure. MSSP BASIC Levels A and B.
- Two-sided (symmetric risk): ACOs share both savings and losses. MSSP ENHANCED Track and ACO REACH.

By organizational lead entity:
- Physician-led ACOs are organized by independent physician groups or independent practice associations (IPAs).
- Hospital-led ACOs are anchored by a health system with employed or affiliated physicians.
- Federally Qualified Health Center (FQHC)-based ACOs use community health center networks as the primary care spine; these intersect with the community health centers and federally qualified health centers framework.

Tradeoffs and tensions

The central tension in ACO design is the savings/risk asymmetry problem: ACOs in early MSSP tracks bear no downside risk, which reduces the financial pressure that motivates behavioral change. Conversely, requiring immediate two-sided risk discourages smaller, independent, or safety-net provider organizations from participating—entities that may serve the highest-need populations.

A second documented tension involves benchmarking erosion. As an ACO successfully reduces costs, its historical benchmark is reset downward in subsequent contract periods, making future savings progressively harder to achieve. This ratchet effect has been identified by MedPAC as a structural disincentive to sustained ACO participation (MedPAC Report to Congress, June 2022).

Attribution versus enrollment creates operational challenges: because Medicare attribution is retrospective and claims-based, ACOs cannot proactively identify their full patient panel at the start of a performance year. This limits the precision of care management interventions and complicates population health analytics.

Quality measure burden is a fourth tension. The MSSP requires reporting across a CMS-defined quality domain set, and the administrative cost of data collection and submission falls disproportionately on smaller practices. The shift toward digital quality measures (dQMs) under CMS's quality strategy aims to reduce manual abstraction burden but requires EHR interoperability infrastructure that not all participating organizations possess.

Common misconceptions

Misconception: Patients must enroll in an ACO to be part of one.
ACO attribution in Medicare is claims-based and retrospective. Beneficiaries do not enroll, sign forms, or receive notification of attribution. This design preserves freedom of choice but means a beneficiary may be attributed to an ACO they have never heard of.

Misconception: ACOs are a form of managed care or HMO.
ACOs do not restrict provider networks or require referrals to stay within a panel. A Medicare beneficiary attributed to an ACO retains full freedom to see any Medicare-participating provider. This distinguishes ACOs structurally from health insurance and medical service coverage models that use narrow networks or gatekeeping.

Misconception: ACOs always save money.
Not all MSSP ACOs generate shared savings. In 2022, approximately 63% of MSSP ACOs generated savings above the minimum savings rate, while 37% did not (CMS MSSP Performance Year 2022 Results). Savings generation is correlated with ACO tenure, organizational type, and regional cost baseline.

Misconception: Value-based care and ACOs are synonymous.
ACOs are one vehicle for value-based payment. Bundled Payments for Care Improvement Advanced (BPCI Advanced), the Comprehensive Primary Care Plus (CPC+) model, and Merit-based Incentive Payment System (MIPS) adjustments under the Quality Payment Program (QPP) are all value-based mechanisms that operate independently of ACO participation.

Checklist or steps (non-advisory)

The following sequence describes the operational phases of MSSP ACO formation and performance, drawn from CMS application and participation requirements (42 C.F.R. Part 425):

1. Eligibility assessment — Confirm that the applying entity meets CMS eligibility criteria: a legal entity formed under applicable state law, capable of receiving and distributing shared savings payments.
2. Beneficiary population estimation — Use historical claims data to estimate the prospective attributed beneficiary pool, typically requiring a minimum of 5,000 assigned Medicare beneficiaries to be eligible for MSSP.
3. Track selection — Evaluate financial risk capacity and select a BASIC Track level (A–E) or ENHANCED Track based on organizational risk tolerance and existing infrastructure.
4. Governance structure establishment — Form an ACO governing body with at least 75% control held by ACO participants (as required by 42 C.F.R. § 425.106), including a mechanism for meaningful provider participation in clinical and operational decisions.
5. Quality reporting infrastructure — Establish data submission processes for CMS-required quality measures, including CAHPS survey administration through a CMS-certified vendor.
6. Compliance and program integrity protocols — Implement policies addressing anti-kickback statute compliance, beneficiary notification (Participation Agreement requirements), and fraud and abuse waivers applicable to ACO arrangements (per OIG/CMS joint guidance).
7. Care management operations — Deploy care coordination staff, risk stratification tools, and care transition protocols targeting high-utilization subpopulations.
8. Annual reporting and reconciliation — Submit quality data by CMS deadlines; receive financial reconciliation reports comparing actual expenditures to benchmark; calculate shared savings or loss liability.

Reference table or matrix

ACO Model Comparison Matrix

Sources: CMS MSSP Participation Agreement Requirements; CMS ACO REACH Model Overview; Oregon Health Authority CCO Framework

Quality Domain Summary (MSSP 2023)

Source: CMS MSSP Quality Measure Specifications

References

• Centers for Medicare & Medicaid Services — Medicare Shared Savings Program
• CMS Innovation Center — ACO REACH Model
• 42 C.F.R. Part 425 — Medicare Shared Savings Program (eCFR)
• 42 U.S.C. § 1395jjj — Affordable Care Act Section 3022 (GovInfo)
• Medicare Payment Advisory Commission (MedPAC) — Reports to Congress
• CMS MSSP Performance Year 2022 Results
• CMS MSSP Fast Facts 2023
• Oregon Health Authority — Coordinated Care Organizations
• CMS Quality Payment Program — Merit-based Incentive Payment System
• [HHS Office of Inspector General — ACO Fraud and Abuse Waivers](https://oig.hhs.gov/compliance/accountable-care-organizations/aco-waivers.asp