Modifier -AG: Primary Physician
Quick reference
- Definition: Modifier -AG identifies the primary physician rendering a professional service billed by a Critical Access Hospital (CAH) electing Method II billing.
- Pairs with: No required pairing modifier, but commonly appears alongside professional service modifiers such as -GC (resident involvement in teaching setting) or assistant-at-surgery modifiers (-80, -81, -82) when applicable.
- Key rule: -AG is used only under CAH Method II reassignment billing — it flags which physician is the primary (not assisting) practitioner on a claim billed under revenue codes 096X, 097X, or 098X on the UB-04.
When to use -AG
Use -AG when:
- The physician has reassigned billing rights to a CAH electing Method II (Optional Payment Method) for outpatient professional services.
- The claim is being submitted on a UB-04 (TOB 85X) under a professional service revenue code (096X/097X/098X), not a CMS-1500.
- The physician is the primary treating/performing physician for the encounter — as distinguished from an assistant surgeon or resident.
When NOT to use -AG (common denials)
- Don’t append -AG to claims billed under CAH Method I, where the physician bills the MAC directly on a CMS-1500 under standard MPFS — -AG has no function outside Method II.
- Don’t append -AG if the practitioner’s reassignment isn’t documented in PECOS; as of January 2, 2026, CMS will return-to-provider (RTP) or deny Method II professional claims lacking correct PECOS reassignment records.
- Don’t append -AG to a claim where the physician is functioning as an assistant at surgery rather than the primary surgeon — that scenario calls for the appropriate assistant-at-surgery modifier instead.
- Don’t append -AG twice on the same claim line, or to non-physician practitioner claims where a different qualifying modifier (e.g., -AH for clinical psychologist, -AJ for clinical social worker) is the correct fit.
Governing framework: CAH Method II billing structure
CMS Medicare Claims Processing Manual, Pub. 100-04, Ch. 4, §250
- Method I (Standard): CAH facility claim goes to the Part A MAC (UB-04, paid at 101% of reasonable cost); the physician bills the Part B MAC separately (CMS-1500, paid under MPFS). No -AG modifier applies here.
- Method II (Optional): The physician/practitioner reassigns billing rights to the CAH. The CAH bills both facility and professional services together on the UB-04 (TOB 85X), with professional services on revenue codes 096X, 097X, or 098X. -AG identifies the primary physician on that combined claim.
- PECOS enrollment requirement: Each reassigning practitioner must have a completed CMS-855I on file, with the reassignment recorded in PECOS. Effective January 2, 2026, claims with undocumented reassignment are subject to RTP/denial.
What you’re paid for with -AG (reimbursement concept)
Under Method II, professional services billed with -AG are reimbursed at 115% of the Medicare Physician Fee Schedule (MPFS) amount, paid to the CAH rather than directly to the individual physician — this reflects Section 1834(g)(2)(B) of the Social Security Act. This is a facility-level payment mechanism, not an add-on RVU modifier like the pricing modifiers you’re used to in profee coding.
Documentation checklist (what has to exist)
To support -AG, the record should show:
- A signed CMS-855I enrollment and active PECOS reassignment record for the billing physician, on file with the CAH.
- Clear documentation identifying the physician as the primary performing/treating provider for the encounter (not an assisting role).
- A CAH attestation confirming the physician will not separately bill the MAC for the same service (prevents duplicate billing / False Claims Act exposure).
- Correct revenue code assignment (096X/097X/098X) and type of bill (85X) on the UB-04 corresponding to the professional service rendered.
Common uses (Urology, Ophthalmology, ENT context)
Since -AG is a facility/CAH billing mechanism rather than a procedure-specific pricing modifier, it shows up less in profee-only shops but is relevant if you code for rural/critical access facilities:
- A urologist performing outpatient cystoscopy services (e.g., 52000) at a CAH under Method II reassignment — the CAH bills the professional component with -AG identifying the urologist as primary physician.
- An ophthalmologist performing minor outpatient procedures at a CAH electing Method II — professional fee reported on the UB-04 with -AG, distinct from any assistant surgeon reporting.
- An ENT physician’s outpatient E/M or minor procedure services at a Method II CAH, where reassignment is active in PECOS and the CAH consolidates facility + professional billing.
- Not applicable in standard hospital-based inpatient profee coding (non-CAH) or CAH Method I settings — you won’t see -AG on typical UHC, Cigna, Aetna, or standard Medicare Part B claims for hospital-employed specialists.
Quick self-check before using -AG
- Is this claim being billed under CAH Method II reassignment, not Method I or standard Part B billing?
- Is the physician the primary performing provider — not an assistant surgeon or resident?
- Is the reassignment documented and active in PECOS (CMS-855I on file)?
- Is the claim using the correct UB-04 / TOB 85X format with the appropriate 096X/097X/098X revenue code, not a CMS-1500?