Alignment Isn't a One-Time Assignment. It's an Ongoing Claims Calculation.

People sometimes talk about a beneficiary being “assigned” to a Medicare Shared Savings Program (MSSP) Accountable Care Organization (ACO) like it's a single event, a name added to a roster at the start of the year. It’s actually closer to an ongoing calculation, and understanding how that calculation actually works matters for any ACO trying to manage who it's accountable for.

The basic mechanism is plurality, not majority. CMS looks at which provider furnished the largest share of a beneficiary's Primary Care Qualified Evaluation and Management, or PQEM, services during a defined lookback window. The provider with the largest share gets the beneficiary assigned to them, even if that share is well under half of the beneficiary's total primary care activity. A beneficiary who splits their care across several providers can still end up assigned to whichever one edges out the others, not necessarily the one they'd consider their main doctor.

The standard assignment window is twelve months, a single calendar year of claims. CMS’s assignment method favors primary care, not a percentage threshold. CMS first checks whether a beneficiary had even one primary care visit with a primary care physician, nurse practitioner, physician assistant, or clinical nurse specialist, inside or outside the ACO, during the assignment window. If so, assignment is based purely on the plurality of primary care services among that group of providers. For beneficiaries who did not visit any of the provider types listed above, CMS counts primary care services furnished by specialists instead. In practice, this means a beneficiary who sees a cardiologist frequently but has genuinely never had a qualifying primary care visit can still end up assigned based on that specialist care, but only because there was no primary care visit to work from in the first place, not because specialist care crossed some threshold.

Starting with performance year 2025, CMS added a third assignment step that looks slightly further back for a narrow group of beneficiaries who had a primary care visit with a nurse practitioner, physician assistant, or clinical nurse specialist during the standard window and at least one primary care visit with a physician in the year before that. For those beneficiaries, CMS looks at an expanded twenty-four-month window covering the standard twelve months plus the twelve months before it. It's a targeted fix for beneficiaries who might otherwise fall through the cracks, not a general shift to a longer lookback for everyone.

Then there's voluntary alignment, which works differently from the methods above. A beneficiary can designate a “primary clinician” directly through Medicare.gov, and that designation takes precedence over whatever the plurality calculation would have produced. The designation must be made by September 30 to count toward the following performance year, and once made, it locks in for two years.

Voluntary alignment is particularly useful for two kinds of beneficiaries. It's useful for a beneficiary who doesn't generate enough claims volume for plurality math to work reliably in the first place, such as a healthy patient who rarely goes to the doctor. The beneficiary’s designation secures assignment on its own without needing a pile of visit data behind it. Voluntary alignment is also useful for a beneficiary who already has a real primary care relationship with a provider but whose plurality could get disrupted by something like an extended skilled nursing facility stay or a stretch of specialist-heavy care. In this case, the beneficiary’s designation protects the relationship regardless of how the claims math shakes out that year. Voluntary alignment isn't good for a blanket push to assign every technically eligible beneficiary. If a beneficiary designates an ACO provider but primarily receives care from other providers, the ACO and designated provider are left with accountability for spending they have limited opportunities to actually affect.

This is why alignment must be actively managed, not set once and left alone. The industry term for losing an assigned beneficiary is leakage, and it happens for reasons that have nothing to do with the quality of care a provider delivers: a SNF stay that shifts the plurality count, a beneficiary who starts seeing a different provider more often, a life change that moves someone's care elsewhere. An ACO that isn't watching its aligned population can lose beneficiaries it never realized were at risk.

None of this is a technical footnote. Alignment is the definition of whose spending and whose outcomes an ACO is being measured against. Get the mechanics wrong, or ignore them entirely, and an ACO can end up accountable for a population whose care it isn’t actually providing.

Nothing in this post constitutes legal, regulatory, or compliance advice. Beneficiary alignment methodology reflects current CMS policy and is subject to change through future rulemaking.