Chiropractic billing modifiers: AT, GA, GY, GZ, 25, and 59 explained.
Six modifiers carry almost all of chiropractic's claim risk, and every one of them makes a specific, checkable claim about the encounter — that the treatment was active, that an ABN is on file, that a service is categorically excluded, that a separate E/M happened, that manual therapy hit a genuinely distinct region. Get one wrong and the outcome usually isn't a downcode, it's an outright denial or an unrecoverable patient balance. This guide covers each modifier's exact trigger, the documentation it has to match, and the specific pattern that gets it flagged on audit.
Key takeaways
- AT is mandatory on every Medicare 98940–98942 line for active care, and it has to match the PART findings in the note — missing it denies the claim outright (CO-4); appending it once the record shows a plateau is upcoding.
- GA and GY are not interchangeable — GA covers a service inside the benefit that's expected to deny this time (needs a signed ABN); GY covers a service that's never covered at all (98943, adjunct therapy). Mixing them up is the most common of the six errors.
- 59/XS on 97140 is a last resort, not a first one. CMS guidance is explicit that the modifier applies only when documentation shows a genuinely separate region — routine, reflexive use is one of the most reliably audited billing patterns in the specialty.
- GZ should almost never appear on a claim you submit on purpose. If a service is expected to deny and there's no ABN, the fix is upstream — get the ABN (GA) or don't bill Medicare for it (GY) — not to submit GZ and hope.
Why these six carry so much weight
Most specialties use modifiers to clarify an edge case. Chiropractic uses them to state, line by line, whether a service is even inside Medicare's benefit and whether the practice is entitled to bill the patient when it isn't. That's a heavier job than most modifier sets carry, and it's why a chiropractic modifier error rarely just adjusts payment — it usually determines whether the claim is payable, appealable, or collectable from the patient at all. Each modifier below makes one factual representation; a payer or a RAC auditor checks that representation against the note, not against the modifier itself.
AT: active/corrective treatment
AT is required on every Medicare line for 98940, 98941, or 98942 billed as active or corrective treatment — care reasonably expected to result in improvement in the patient's condition, or necessary to establish a diagnostic baseline early in a course of care. It is not optional documentation flavor; its absence causes the claim to deny outright as CO-4, not to pay at a reduced rate. Note what AT does not certify: it doesn't certify that a manipulation occurred (the CPT code itself does that) — it certifies the clinical trajectory the treatment is aimed at.
The audit trigger: AT appended without supporting PART findings. Medicare's chiropractic articles require at least 2 of the 4 PART elements (Pain/tenderness, Asymmetry, Range of motion, Tissue/tone), with at least one being A or R, documented at the visit — not established once at intake and assumed to carry forward. A note that reads "continue care, AT" with no region-specific PART findings for that date of service supports the CPT code but not the modifier. Auditors pull a sample of AT-modified claims and check each one against that visit's documented findings; a pattern of AT on every visit regardless of what the note shows is exactly what a payer's SIU or a Medicare RAC review is built to catch.
| Visit stage | What the note needs | Modifier |
|---|---|---|
| Early course of care, diagnosis being established | PART findings supporting the region(s) billed; treatment plan with expected outcome | AT |
| Mid-course, condition improving | PART findings showing measurable change from a prior visit — decreasing pain, expanding ROM | AT |
| Condition plateaued, no further expected improvement | Note documents the plateau explicitly; ABN signed before the next visit | GA (not AT) |
GA: ABN on file, expected maintenance-care denial
GA replaces AT the moment a course of care crosses from active/corrective treatment into maintenance — when the documentation no longer supports expected improvement in the patient's condition, but the patient continues to receive care to preserve their current status. GA signals that a properly executed, service-specific Advance Beneficiary Notice was signed before the visit and that the expected denial should post as patient financial responsibility (PR-204), not a payer dispute.
The audit trigger the task calls out specifically: GA used on the wrong code. GA belongs on the CMT line (98940–98942) once maintenance status is documented — it does not belong on 98943 or on any adjunct therapy code, because those aren't "sometimes covered, sometimes not" services; they're excluded from the chiropractic benefit categorically, every time, for every patient. That's GY's job, not GA's. The reverse mismatch happens too: a practice appends GY to a maintenance-care CMT line because staff conflate "this is going to deny" with "this is never covered," when the CMT itself is squarely inside the benefit — it's just not medically necessary on this date of service. Both denial paths end in a denial either way, but the wrong modifier on the CMT line means the ABN-backed patient liability doesn't post the way it should, and secondary payers and patient statements downstream get confusing, hard-to-reconcile denial reasons.
A GA claim without a valid ABN on file doesn't protect the practice at all — the claim can still deny with the practice unable to collect from the patient. The ABN has to be specific to the service, signed before the service is rendered, and kept on file; a blanket, undated, or after-the-fact ABN doesn't satisfy the requirement on audit.
GY: statutorily excluded service
GY marks a service as excluded from Medicare coverage by statute, not by a medical-necessity judgment call. In chiropractic, that's 98943 (extraspinal manipulation) and every non-CMT service a chiropractor bills — the timed adjunct codes (97110, 97112, 97140, 97530), unattended e-stim (G0283), spinal x-rays billed as part of chiropractic care, and E/M services beyond what the benefit allows. None of these become covered no matter how well-documented or medically appropriate they are, because the chiropractic benefit itself is limited by statute to manual manipulation of the spine to correct a subluxation.
GY is rarely audited on its own — there's no documentation standard to fail, because the service was never going to be covered regardless of the note. The practical risk is upstream: omitting GY entirely and billing these lines with no modifier produces a confusing, generic denial that complicates secondary insurance billing and patient statements. Append GY proactively on every excluded line rather than waiting for the denial to arrive unmodified.
GZ: expected denial, no ABN
GZ tells the payer a service is expected to be denied and no ABN was obtained. Structurally it exists for situations where the ABN process broke down — and functionally, it should almost never appear on a claim your practice submits on purpose. If you know in advance that a service is going to be denied, you have exactly two correct paths: get a signed, service-specific ABN before the visit and bill with GA, or recognize the service as categorically excluded and bill with GY. GZ is what's left over when neither of those happened, and its defining consequence is that it waives patient liability — you cannot bill the patient after the fact for a GZ-modified line.
Frequent, intentional use of GZ is a compliance flag in its own right: it tells a payer or auditor that the practice routinely bills services it already expects to fail, without the ABN workflow that would let it collect for them. If GZ is showing up on your claims regularly, the fix is a process fix — catch the maintenance-care transition or the categorical exclusion before the visit, not after the remittance comes back.
Modifier 25: same-day E/M
Modifier 25 marks a significant, separately identifiable E/M service on the same day as CMT. It's scrutinized harder in chiropractic than in most specialties because payers assume routine pre-adjustment reassessment — asking how the patient's been feeling, a quick PART recheck before manipulating — is already bundled into the CMT code, not a separate billable service. Three scenarios support it: a genuinely new complaint unrelated to the condition already being treated, a documented exacerbation or flare that changes the clinical picture, or a formal re-evaluation of the treatment plan at a defined interval. Everything else is part of the CMT service.
What the note has to show: a distinct history and exam addressing the new or changed complaint, and medical decision making that's clearly separable from "decide whether to adjust today" — ideally in a physically separate section of the note rather than woven into the CMT documentation. The full time-vs-MDM leveling grid and a documentation template built for this specific scrutiny live in our chiropractic E/M and modifier 25 guide.
59 / XS: unbundling 97140 from CMT
97140 is a Column 2 code bundled into the CMT codes under NCCI Procedure-to-Procedure edits whenever both are performed in the same anatomic region during the same encounter — the logic being that manual therapy technique overlaps substantially with manipulation when applied to the same region. Modifier 59, or the more specific XS (separate structure), overrides the bundle, but only when the manual therapy targeted a region genuinely distinct from the region(s) adjusted, with its own clinical findings and its own timed-minutes documentation apart from the CMT note.
The audit trigger the task calls out specifically, and it's the single most reversed pattern in a chiropractic NCCI audit: 59 used as a first resort instead of last resort. CMS's own modifier-59 guidance is explicit that it should be appended only when no more descriptive, specific modifier applies and the record independently supports a distinct service — not as a default override reached for whenever a same-day 97140/CMT pair needs to clear a bundling edit. A defensible pattern: CMT to the cervical and thoracic spine (98941, AT) plus 97140 to the ipsilateral shoulder girdle for a documented soft-tissue restriction unrelated to the spinal adjustment. An indefensible pattern: CMT to the lumbar spine plus "manual therapy to the lumbar paraspinals" with no finding beyond what already supports the adjustment — same region, same rationale, and no modifier fixes that.
| Modifier | Asserts | Audit trigger |
|---|---|---|
AT | Treatment is active/corrective, expected to improve the condition | Appended with no PART findings supporting it that date, or continued after the note shows a plateau |
GA | Signed ABN on file, service inside the benefit but expected to deny (maintenance) | Applied to a categorically excluded code instead of the maintenance-care CMT line, or applied with no valid ABN on file |
GY | Service is never covered under the chiropractic benefit, by statute | Omitted entirely, producing a confusing unmodified denial instead of a clean statutory-exclusion denial |
GZ | Expected denial, no ABN — patient cannot be billed | Appearing routinely at all, which signals the ABN or exclusion workflow is breaking down upstream |
25 | A separately identifiable E/M happened the same day as CMT | Applied to a routine pre-adjustment check-in with no distinct history/exam/MDM in the note |
59 / XS | 97140 targeted a genuinely separate region from the CMT adjustment | Applied reflexively to every same-day 97140/CMT pair regardless of region, rather than as a documented last resort |
Do and don't
- Match AT to that visit's documented PART findings, not to a default template value.
- Get a service-specific ABN signed before a maintenance-care visit, then switch to GA going forward.
- Append GY proactively on every extraspinal and adjunct-therapy line billed to Medicare.
- Treat 59/XS as a last resort, and document the separate region and separate timed minutes when you use it.
- Don't keep billing AT once the note itself documents a plateau in the patient's condition.
- Don't confuse GA (expected-to-deny, ABN required) with GY (never covered, no ABN needed).
- Don't submit GZ as a routine way to bill a service you already know won't be covered.
- Don't apply modifier 25 to a routine pre-adjustment reassessment with no separable MDM.
Not sure your chiropractic modifier logic is right?
We'll audit a sample of your recent claims for AT/GA/GY mismatches, reflexive 59 use, and modifier 25 patterns, and show what's recoverable.
Frequently asked questions
What's the practical difference between GA and GY on a chiropractic claim?
GA says the service is inside the Medicare chiropractic benefit category but is expected to be denied this time — usually because care has become maintenance rather than active treatment — and a signed ABN is on file to shift the balance to the patient. GY says the service is never inside the benefit category at all, like 98943 extraspinal manipulation or any adjunct modality, so no ABN is required and the denial is automatic. Appending GA to a 98943 line, or GY to a maintenance-care CMT line, is a common mismatch that confuses secondary billing even though the claim still denies either way.
Is modifier 59 required every time 97140 is billed with a CMT code?
No — only when the manual therapy genuinely targeted a separate anatomic region from the region or regions adjusted, with its own documented findings and its own timed minutes. CMS guidance treats 59 and the X-modifiers as a last resort, appended only when no more specific way to describe the distinct service exists, not a default override applied so the second line pays. Appending it as a first resort on every same-day 97140/CMT pair, regardless of region, is one of the most reliably flagged patterns in a chiropractic NCCI audit.
Can we bill modifier 25 on a routine adjustment visit?
Not for the routine pre-adjustment check-in that happens at nearly every visit — payers treat that reassessment as part of the CMT code itself, not a separately billable E/M. Modifier 25 is defensible only for a genuinely new complaint, a documented exacerbation that changes the clinical picture, or a formal treatment-plan re-evaluation at a defined interval, and the note has to show medical decision making that's clearly separable from the decision to adjust that day.
Verify before billing. CPT is a registered trademark of the American Medical Association; codes here are paraphrased, not reproduced from the CPT Professional edition. CPT, HCPCS and ICD-10 codes, coverage policy, and bundling edits change, including annual code-set updates. This page reflects standard industry practice and is provided for general education — it is not a substitute for your own compliance review, your current payer contracts, or the current-year code sets. Confirm requirements against your specific payer mix before submitting claims.