ACO Improvement Act Tweaks Medicare Rules: Who Your Doctor Counts For Starting 2027

The ACO Assignment Improvement Act of 2025 is a technical bill focused entirely on adjusting how Medicare links beneficiaries to Accountable Care Organizations (ACOs) within the Medicare Shared Savings Program (MSSP). Think of it as updating the algorithm that decides which healthcare team gets credit—and accountability—for your care.

The Algorithm Update: More Doctors Count

Starting with the performance year beginning January 1, 2027, this bill changes the rules for beneficiary assignment. Currently, Medicare uses primary care services provided by certain doctors to figure out which ACO you’re assigned to. This new law broadens that scope. It specifically allows primary care services provided by a wider group of “certain ACO professionals” (referenced in a specific section of the Social Security Act) to count toward linking you to an ACO for performance measurement. The goal is to make the assignment process more accurate by recognizing more of the providers who are actually managing your primary care. It’s a clean-up job that acknowledges that primary care isn't just delivered by one specific type of provider anymore.

What This Means for Your Annual Check-Up

If you're a Medicare beneficiary, this change is mostly happening behind the scenes, but it matters for the quality of your care. ACOs are groups of doctors, hospitals, and other providers who agree to work together to give you coordinated, high-quality care. If they save Medicare money while meeting quality targets, they share in the savings. But to measure their performance, Medicare has to assign patients to them.

By including more types of primary care professionals in the assignment calculation, the bill aims to give ACOs a more accurate picture of the population they are responsible for. For example, if you see a specialized nurse practitioner or physician assistant for your routine care, and they work within an ACO, their services might now count toward assigning you to that ACO. This makes the ACO more accountable for your outcomes, which is the whole point of the program. If they're being measured on your health, they have a stronger incentive to coordinate your care.

The Fine Print and the Future

While the bill is largely administrative and includes some minor technical clean-up edits to existing law, the core change is the expansion of eligible primary care professionals. The downside right now is that the bill text refers to these eligible professionals using a specific statutory citation (subsection (h)(1)(B) of the relevant statute) without defining them here. We have to look up that section to know exactly which providers are being added to the mix. This vagueness means that ACOs and providers need to wait for the specific regulation updates to fully understand the impact on their patient panels and their bottom line.

For the healthcare systems themselves, particularly ACOs, this means their patient attribution models will shift in 2027. Some ACOs might find their assigned patient population changes significantly, which directly affects their performance metrics and potential shared savings. It’s a necessary technical adjustment to keep the MSSP program functional and fair, but any change to the attribution algorithm means a period of adjustment for the organizations trying to hit their targets.