ACOs for a practice of one: what joining an ACO changes

Can a solo practice join an ACO?

Yes. Accountable care organizations are not limited to large groups, and the common path for a solo or small practice is to join an existing ACO rather than build one from scratch. You keep your own practice, your own enrollment, and your own patients; you add a layer of shared accountability for the cost and quality of a Medicare population. The value-based arrangements this belongs to are the direction CMS has been steering Medicare through its Innovation Center 1Ref 1Centers for Medicare & Medicaid Services (2026).CMS Innovation Center.That the CMS Innovation Center tests the alternative payment models that value-based arrangements such as accountable care trace to — used only for landscape context, not for any specific Shared Savings Program rule..

The harder question is the second one — should you. That is a business decision with a few concrete inputs rather than a yes-or-no rule, and most of the confusion around it comes from misunderstanding what actually changes when you join. Very little of your day-to-day billing does; most of the change lands in reporting and in what you are measured on.

What an ACO actually is

An ACO is a group of providers that agrees to be jointly accountable for the quality and total cost of care for a defined population, and to share in the savings — and, depending on the arrangement, the losses — if it meets its targets. It is a payment model layered on top of ordinary Medicare, not a different insurance product. These models trace back to the alternative-payment experiments CMS runs through its Innovation Center 1Ref 1Centers for Medicare & Medicaid Services (2026).CMS Innovation Center.That the CMS Innovation Center tests the alternative payment models that value-based arrangements such as accountable care trace to — used only for landscape context, not for any specific Shared Savings Program rule..

Medicare's permanent version is the Shared Savings Program, and its specifics — the participation tracks, how savings are calculated, the benchmarks, and the minimum size — are set by CMS and revised over time. Because those details move and are program-specific, treat this page as the concept and the decision, and pull the current program rules from CMS's own Shared Savings Program materials rather than from a recruiter's summary.

You keep billing fee-for-service

The biggest misconception is that joining an ACO changes how you get paid for a visit. It does not. You continue to submit claims to your Medicare contractor and are paid under the physician fee schedule exactly as before 2Ref 2Centers for Medicare & Medicaid Services (2026).Physician Fee Schedule.That the Medicare Physician Fee Schedule sets payment for clinician services and is updated annually through rulemaking, so an ACO participant's per-service revenue still runs on the fee schedule and its yearly conversion-factor reset.3Ref 3Centers for Medicare & Medicaid Services (2026).Medicare Administrative Contractors.That Medicare claims are administered by regional MACs, which is where an ACO participant still submits and is paid on ordinary fee-for-service claims.. The shared-savings arrangement settles separately and after the fact, based on how the whole ACO performed against its cost and quality targets — it sits on top of your claims, not in place of them.

That separation matters for cash flow: your ordinary Medicare payments keep arriving on their normal cadence, and any shared-savings distribution is a distinct, later event that is never guaranteed. It also means the fundamentals you already track — your fee-schedules and how RVUs drive them, the conversion factor that resets each year — still govern your actual per-service revenue inside an ACO just as they do outside one.

What changes is your quality reporting

The real operational change is how you satisfy Medicare's quality-reporting expectations. For most eligible clinicians, that obligation is defined through MIPS, which scores quality, cost, improvement activities, and interoperability and adjusts Part B payment accordingly 4Ref 4Centers for Medicare & Medicaid Services (2026).MIPS Overview.That MIPS scores eligible clinicians on quality, cost, improvement activities, and interoperability and adjusts Part B payment, and that clinicians under the low-volume threshold are excluded — the reporting obligation an ACO's advanced-payment-model pathway shifts.. Participating in an ACO generally shifts you onto the program's advanced-payment-model reporting pathway instead of reporting as an individual clinician.

For a solo practice, that shift is often the single biggest draw: individual MIPS reporting is a real administrative burden for a practice of one, and moving quality reporting into the ACO can lighten it. This is where measurement-based care as a solo differentiator stops being only a clinical idea and becomes an administrative one — the ACO turns the data you already gather into the reporting it handles on your behalf.

The low-volume threshold changes the math

Before you count reporting relief as a benefit, check whether you owe any reporting at all. Clinicians below the MIPS low-volume threshold are excluded from MIPS entirely 4Ref 4Centers for Medicare & Medicaid Services (2026).MIPS Overview.That MIPS scores eligible clinicians on quality, cost, improvement activities, and interoperability and adjusts Part B payment, and that clinicians under the low-volume threshold are excluded — the reporting obligation an ACO's advanced-payment-model pathway shifts., which means a low-Medicare-volume solo may already have no individual reporting burden for an ACO to relieve. Measure your Medicare volume against the current threshold first; it decides whether this whole benefit even applies to you.

This is the step most easily skipped and most consequential. A practice that sees very few Medicare patients gets little reporting relief from joining, because it had little obligation to begin with — and the administrative and data-sharing cost of ACO membership doesn't shrink to match. The relief argument is strongest for a solo with enough Medicare volume to be squarely inside MIPS.

The measures you'll be judged on

Inside an ACO you are measured on population quality, not just your own claims. The measure sets Medicare and commercial plans report from — HEDIS chief among them — include several that reach directly into outpatient practice patterns, such as antidepressant medication management and follow-up after an emergency visit for mental illness 5Ref 5National Committee for Quality Assurance (2026).HEDIS.That HEDIS is the measure set plans report, and that several measures — antidepressant medication management and follow-up after an emergency visit for mental illness — reach into outpatient behavioral-health practice patterns an ACO participant is measured on.. Even a behavioral-health or primary-care solo will find its own patients reflected in those measures.

That is worth knowing before you sign, because the measures shape what the ACO will ask of you: closing care gaps, documenting follow-up, tracking outcomes at solo scale. If those align with how you already practice, membership is low friction. If they don't, the ACO's expectations become ongoing work. The commercial parallel — commercial quality bonuses tied to similar measures — often runs alongside the Medicare arrangement.

Should a practice of one join?

The decision comes down to a handful of factors you can actually weigh, not a blanket recommendation. Run each one against your own practice before you take a position, and read the specific ACO's participation agreement rather than a general description — the terms vary by ACO.

• Your Medicare volume, and whether it puts you inside or below the MIPS low-volume threshold.
• Whether the arrangement is upside-only or carries downside risk, since that changes what a bad year costs you.
• The reporting and data-sharing the ACO requires, against the administrative capacity of a practice of one.
• How closely the ACO's quality measures match how you already practice.
• The current Shared Savings Program rules from CMS, and the ACO's own numbers, reviewed with your accountant before you commit.

None of this depends on your Medicare enrollment status changing — an ACO doesn't alter whether an 855B for a practice of one or your existing enrollment is what put you in Medicare. It layers on top of what you already have.