The ACO (MSSP)
480 Experiments in Keeping Medicare Patients Healthy
Here’s the basic deal: a group of providers — let’s say a health system, 200 primary care physicians, and a handful of post-acute partners — forms an ACO. Medicare assigns them a benchmark: last year, the patients attributed to this group cost Medicare $12,000 per person. If the ACO can keep costs below $12,000 while hitting quality targets, it keeps a share of the savings.
That’s it. That’s the ACO.
What It Is
An Accountable Care Organization is a group of providers that voluntarily takes shared accountability for the quality and total cost of care for a defined Medicare population. About 480 MSSP ACOs cover 11+ million Medicare beneficiaries.
The ACO itself is typically an LLC that sits alongside the participating provider organizations. It doesn’t own hospitals or practices — it coordinates contracts, quality reporting, and shared savings distribution.
Why It Exists
The ACA created ACOs in 2010 as the primary vehicle for moving Medicare from fee-for-service to value. The logic: if providers share in the savings from reducing unnecessary utilization and keeping patients healthy, they’ll invest in prevention and care coordination.
How It’s Organized
The ACO contracts with CMS and is responsible for a population of attributed beneficiaries. “Attribution” means Medicare looks at which primary care doctor a patient saw most often and assigns that patient to the doctor’s ACO.
Inside the ACO, the actual work happens through care managers, data analytics teams, and clinical protocols that try to prevent ER visits, reduce hospital readmissions, and manage chronic disease. The organizational infrastructure that does this work is often a CIN.
The Tradeoffs
The upside: Shifts incentives toward health. Encourages investment in prevention and care management. Can be built on existing structures.
The downside: Shared savings alone may not fund the transformation needed. Attribution is imperfect — patients can see any doctor they want, but the ACO is accountable for their costs. Administrative burden is significant.
The Bottom Line
ACOs are the most important structural experiment in Medicare. They’re not revolutionary — they’re evolutionary, layering shared accountability on top of fee-for-service. The results are modest but real: MSSP ACOs have generated billions in cumulative savings. The question is whether modest savings are enough to justify the complexity.