CMS is changing how Medicare Shared Savings Program (MSSP) beneficiary assignment works — but not all at once. For performance year (PY) 2027, CMS expands the primary-care-services definition used in assignment to include SBIRT, vaccine-adverse-effects management, and advance care planning codes.
Two broader changes — excluding certain non-ACO TIN charges and revising the Medicare enrollment eligibility test — do not take effect until PY 2028.
The CY 2027 PFS final rule (CMS-1848-F), effective January 1, 2027, finalizes all three changes with a staggered timeline. This article covers:
- Which primary care services were added for PY 2027 assignment
- Why the timing distinction between 2027 and 2028 prevents planning errors
- What the revised Medicare enrollment test changes about beneficiary eligibility
- How non-ACO TIN charges get treated differently starting PY 2028
- What these changes could mean for ACO assignment outcomes
TLDR — ACO ASSIGNMENT CHANGES
Three changes, two timelines — don’t plan them as one.
- 📅 PY 2027 — SBIRT, vaccine adverse-effects, and ACP codes added to assignment primary-care-services list
- 📅 PY 2028 — Certain non-ACO TIN charges excluded from assignment comparison
- 📅 PY 2028 — Revised enrollment test (one qualifying month with Parts A+B and no group health plan)
- 🔗 New codes include G2011, G0396, G0397, G0576, G0572, and G0573
- ⚠️ Adding a service to the assignment list does not automatically assign the patient to the ACO
How does MSSP beneficiary assignment work?
Before getting into the changes, the mechanism deserves a short explanation. MSSP assignment identifies the Medicare fee-for-service (FFS) beneficiaries for whom an ACO is financially and clinically accountable.
CMS compares a beneficiary’s allowed charges for qualifying primary care services across ACO and non-ACO billing entities through a stepwise claims-based methodology.
The assigned population affects:
- Financial benchmarking
- Performance assessment
- Quality reporting
Assignment hinges on where the beneficiary received the plurality of their qualifying primary care services.
That plurality comparison is what makes the definition of “primary care services” so consequential — adding or removing codes from the list changes which claims count toward the assignment calculation.
MSSP assignment concerns Original Medicare FFS beneficiaries. Medicare Advantage enrollment is relevant because CMS treats it as Medicare group health plan enrollment when applying the eligibility criteria.
What primary care services were added for PY 2027?
CMS expanded the primary-care-services definition used in assignment beginning with PY 2027. Six HCPCS codes covering three service categories now count toward the assignment methodology.
| Service category | HCPCS codes |
| Screening, Brief Intervention, and Referral to Treatment (SBIRT) | G2011, G0396, G0397 |
| Vaccine adverse-effects management | G0576 |
| Advance care planning | G0572, G0573 |
The advance care planning codes (G0572 and G0573) are the same new clinical-staff ACP codes finalized for 2027 billing.
G0576 replaces the proposed placeholder GADV1. The SBIRT codes were already in the Medicare system — CMS is adding them to the assignment list, not creating them.
What the addition does and does not do
Adding a service to the assignment definition means CMS can count claims for that service when calculating which provider had the plurality of a beneficiary’s primary care.
It does not mean that every claim for SBIRT, vaccine adverse-effects management, or ACP automatically assigns the patient to the billing ACO.
CMS still applies:
- Assignment eligibility criteria
- The stepwise methodology for comparing primary care service use
- The plurality calculation across ACO and non-ACO entities
In practical terms, an ACO whose clinicians furnish a high volume of SBIRT or ACP services may see more of those services counted in the assignment calculation — which could shift which entity ends up with the plurality. But “could shift” is not “will assign.”
How will non-ACO TIN charges be treated starting PY 2028?
This change targets a specific billing pattern. When an ACO professional bills qualifying primary care services through both a participant TIN (inside the ACO) and a non-participant TIN (outside the ACO), the dual billing can distort the plurality comparison.
Starting PY 2028, CMS excludes from the assignment calculation allowed charges for primary care services that meet all three conditions:
- Furnished by an ACO professional used in assignment
- Billed under a Medicare-enrolled TIN
- Billed under a TIN not participating in any ACO
How this works in practice
Consider a physician who practices at an ACO-participating clinic three days a week and at a separate, non-ACO urgent care center two days a week.
Under the current rules, primary care services billed through both TINs count in the assignment methodology. A beneficiary who sees the physician at both locations has those charges split across two TINs — potentially diluting the ACO’s claim to the plurality.
Under the PY 2028 rule, CMS removes the non-ACO TIN charges from the comparison. Only the charges billed through the ACO’s participant TIN count toward assignment for that professional.
Boundaries of the rule
The exclusion is narrowly drawn:
- It applies only to charges from an ACO professional billing through a non-ACO TIN
- It does not exclude all care from non-ACO providers
- It does not guarantee that the beneficiary gets assigned to the ACO
CMS describes the policy as reducing the chance that one professional’s billing patterns inside and outside an ACO affect assignment in a way that financially advantages the ACO. The effect is cleaner data in the plurality calculation — not an automatic assignment bump.
What changes in the Medicare enrollment eligibility test for PY 2028?
The enrollment eligibility test determines which beneficiaries can be assigned to an MSSP ACO. CMS revised this test for PY 2028 and later to make more beneficiaries potentially eligible.
Prior test
Under the prior rules, a beneficiary generally needed at least one month of Part A and Part B enrollment during the 12-month assignment window — without any month in that window involving Part A-only, Part B-only, or Medicare group health plan enrollment (including Medicare Advantage).
A single month of MA enrollment during the 12-month window could disqualify the beneficiary entirely, even if the remaining 11 months were standard FFS coverage with both parts.
New test (PY 2028)
CMS now asks whether the beneficiary has at least one month during the 12-month assignment window with:
- Both Part A and Part B enrollment
- No Medicare group health plan enrollment (including MA) during that same month
The test shifts from a whole-window screen to a month-level check. A beneficiary who has 10 months of MA coverage and 2 months of Parts A and B FFS coverage can qualify — as long as at least one of those FFS months has no overlapping group health plan enrollment.
ENROLLMENT TEST — BEFORE VS. AFTER PY 2028
Prior test
One month of MA enrollment anywhere in the 12-month window could disqualify the entire beneficiary
New test (PY 2028)
Only needs one month with both Parts A+B and no group health plan enrollment in that same month
Prospective assignment implications
CMS also revised the prospective assignment exclusion criteria for PY 2028 to reflect the new enrollment test. ACOs using prospective assignment should expect the updated exclusion rules to apply when CMS refreshes the prospective assignment list during the performance year.
Why does the 2027 versus 2028 timing distinction matter?
Describing all three changes as “2027 assignment changes” would be inaccurate — and could lead to operational planning errors.
| Change | Effective performance year |
| Expanded primary-care-services definition (SBIRT, vaccine-effects, ACP codes) | PY 2027 |
| Non-ACO TIN charge exclusion from assignment | PY 2028 |
| Revised Medicare enrollment eligibility test | PY 2028 |
An ACO that builds its PY 2027 assignment analytics around the enrollment-test change or the TIN exclusion will be working against rules that do not apply yet. The primary-care-services expansion is the only piece active for PY 2027.
The practical risk is in reporting and financial modeling. ACOs that project PY 2027 assignment using PY 2028 rules will overestimate their assigned population. Benchmarking errors downstream are harder to unwind than they are to prevent.
What could these changes mean for ACO assignment outcomes?
CMS’s stated aim is to increase the number of Medicare beneficiaries in accountable care relationships. The three changes pursue that goal through different mechanisms:
- The expanded service list captures more primary care touchpoints in the assignment calculation
- The TIN exclusion sharpens the plurality comparison by removing split-billing noise
- The relaxed enrollment test makes more beneficiaries eligible for assignment
None of these changes guarantees that a specific ACO will see a larger assigned population.
CMS still applies the full assignment methodology — including eligibility criteria, the stepwise comparison, and the plurality threshold. The changes alter the inputs to that methodology, not its outcomes.
For ACOs with professionals who bill through both participant and non-participant TINs, the PY 2028 TIN exclusion may have the most visible effect.
The enrollment-test change could matter for ACOs in markets with high MA penetration, where beneficiaries cycle between Original Medicare and MA coverage during the year.
What should ACOs do to prepare?
Operational preparation tracks the staggered timeline.
Update Claims and Assignment Analytics
Add the new primary-care-service codes to claims tracking and beneficiary assignment analytics.
Verify EHR and Billing Code Accuracy
Confirm that systems correctly capture and code SBIRT, vaccine-adverse-effects, and advance care planning (ACP) services.
Review Dual-TIN Billing
Identify ACO professionals who submit claims through both participant and non-participant TINs.
Model Assignment Impact
Estimate how excluding charges from non-ACO TINs could change beneficiary assignment outcomes.
Prepare for the Revised Enrollment Test
Track beneficiary Part A and Part B status separately from group health plan enrollment, month by month.
Assess Prospective Assignment
Evaluate the potential effects on beneficiary assignment if the ACO elects the prospective assignment option.
Assignment changes make a difference only if the claims feeding them are clean
MSSP beneficiary assignment depends on accurate claims for qualifying primary care services.
A miscoded SBIRT encounter, a missed ACP time attribution, or a credentialing gap that prevents a provider from billing under the right TIN can all affect the assignment calculation before anyone notices.
MedHeave works inside your EHR and billing systems to get it right the first time.
- Claim submission within 24-48 hours with payer-specific scrubbing
- Dedicated account managers with direct lines — no rotating support queues
- Credentialing and enrollment management across multiple states and payers
- Coding accuracy reviews covering CPT, HCPCS, ICD-10, and modifier validation
- Denial management with documented 72-hour response and 80%+ overturn rate
Frequently asked questions
Here are some commonly asked questions on this topic:
Only the primary-care-services definition expansion starts in PY 2027 — adding SBIRT codes (G2011, G0396, G0397), a vaccine-adverse-effects code (G0576), and advance care planning codes (G0572, G0573) to the assignment list. The non-ACO TIN charge exclusion and the revised Medicare enrollment eligibility test both start in PY 2028. Describing all three as “2027 changes” would be inaccurate and could cause planning errors.
No. Adding codes to the primary-care-services definition means CMS counts those services in the plurality comparison — the stepwise process that determines which entity furnished the largest share of a beneficiary’s qualifying primary care. Assignment still depends on eligibility criteria, the plurality calculation, and the full methodology. A single ACP or SBIRT claim does not trigger automatic assignment.
Starting PY 2028, CMS removes primary care charges from the assignment comparison when an ACO professional bills those services through a TIN not participating in any ACO. The exclusion applies only to that professional’s non-ACO TIN charges — not to all care from non-ACO providers. CMS describes the policy as cleaning up the plurality calculation when a physician practices in both ACO and non-ACO settings.
No. MSSP assignment concerns Original Medicare FFS beneficiaries. The revised enrollment test asks whether a beneficiary has at least one month with both Parts A and B and no Medicare group health plan enrollment (including MA) during that month. A beneficiary enrolled in MA for the full 12-month window would not have a qualifying month. The change makes partial-year FFS beneficiaries more accessible — it does not pull MA-enrolled beneficiaries into MSSP.
Beneficiary assignment feeds the ACO’s financial benchmarking and performance assessment. If the expanded service list, TIN exclusion, or enrollment test brings more beneficiaries into the assigned population, that changes the denominator for benchmark calculations. ACOs should model assignment changes against their financial projections — but should not assume specific outcomes until they see the actual claims data under the new rules.
