Medicare Chiropractic Billing: AT vs GA Modifiers, ABNs, and Maintenance Care

Medicare is the payer that punishes ambiguity. It asks you to declare, on every single spinal manipulation line, what kind of care you just delivered — and then holds you to it. I’m a practicing chiropractor, and no billing topic has cost me and my colleagues more time than the small cluster of decisions in this guide: what the benefit actually covers, when a patient crosses from active care into maintenance, which modifier says so, and what has to be signed before that visit happens. None of it is difficult. All of it is unforgiving.

What Medicare’s chiropractic benefit actually covers

Start here, because almost every downstream mistake is a misunderstanding of scope. Medicare’s chiropractic benefit covers manual manipulation of the spine to correct a subluxation. That is the benefit — in practice, the chiropractic manipulative treatment codes 98940, 98941, and 98942, distinguished by how many spinal regions you treated.

And that is allit covers when the service is provided by a chiropractor. Examinations, x-rays you take yourself, modalities, therapeutic procedures — billed under the chiropractic benefit, they are excluded by statute. Medicare never pays for them in that context, no matter how medically appropriate they were or how well you documented them.

That matters because the two kinds of “no” behave completely differently. A statutory exclusion is never payable and no amount of documentation changes it. A medical-necessity denial— the kind maintenance care gets — is a judgment about this patient’s currentcare, and it’s the one your chart and your modifiers can speak to.

Active/corrective care vs maintenance care

This single distinction drives every modifier, every ABN, and most audit findings in chiropractic. Active or corrective treatmentis care that is expected to produce meaningful improvement in the patient’s condition or function. Maintenance careis care delivered once further clinical improvement can no longer reasonably be expected — care that supports and preserves the patient’s current state rather than correcting toward a better one.

Notice what maintenance care is not. It is not bad care and it is not a failure of the treatment plan — plenty of patients genuinely benefit from ongoing supportive care. It is simply not a covered service, and the entire compliance problem is that the transition happens quietly, inside a treatment relationship that feels continuous, while the billing keeps doing what it did last month.

So: the phase change is a clinical decision you make and document on a specific date, not a status that drifts. The modifier, the ABN, and the patient’s financial expectation all follow from that one documented judgment.

The modifiers: AT, GA, and GZ

Every Medicare spinal CMT line has to declare its treatment character. A bare 98940–98942 line carrying none of these modifiers reads as unstated maintenance and mis-assigns financial liability.

AT — active treatment

ATsays: this is active, corrective care, and the record behind it supports that. The inverse is the part practices get wrong — AT must not appear on a maintenance visit. The modifier is an assertion about the care, and appending it out of habit is how a documentation issue becomes a repayment issue.

GA — maintenance with a signed ABN on file

When the care is maintenance and the patient has signed an Advance Beneficiary Notice covering that date of service, GAis the modifier. It tells Medicare: I expect this to deny, the patient was told in advance, and the patient accepted responsibility. The claim goes in, the denial comes back, and because the waiver of liability is on file, the balance is legitimately the patient’s.

GZ — expected denial, no ABN

GZis the honest version of a mistake: you expect the service to be denied as not reasonable and necessary, but you have no signed ABN covering it. It is accepted on a claim and occasionally the right call — but it is not a substitute for GA. Without the signed waiver, liability never shifted to the patient, so the denial lands on the practice. Every GZ should prompt the same question: why wasn’t the conversation had before the visit?

ABNs: when they’re required, and the timing that makes them valid

Which denial you’re facing decides whether the notice does any work

Go back to the two kinds of “no.” For maintenance manipulation— a medical-necessity denial — the ABN is what makes the patient balance collectible. If you intend to keep treating and keep billing, that signature isn’t optional paperwork; it’s the mechanism. For services statutorily excludedwhen a chiropractor provides them, the patient is already responsible — the notice isn’t what creates liability, though many practices issue one voluntarily so nobody is surprised by a bill.

Those are two different reasons, and one notice that blurs them serves neither. If a visit includes both maintenance manipulation and a statutorily excluded service, splitting into separate notices beats one vague sentence covering both.

Timing: before the care, covering the care

Two timing rules do nearly all the work, and both are about the calendar rather than the form.

• Signed before the service.An ABN signed after the visit does not retroactively cover it. “Advance” is the operative word — far enough ahead that the patient can consider the options, ask what the care costs, and decline. A signature collected on the way out is not advance notice.
• Covering that date of service.A signed ABN covers services from its signed date through the course-of-treatment end date written on the form. A visit outside that window isn’t covered, and billing the patient for that denial is contestable. Practices track the signature and forget the window — the window is the part that expires.

The form itself

Use the official form — CMS-R-131, the Advance Beneficiary Notice of Non-coverage (with CMS-R-131-Sas its Spanish counterpart). A homemade “you may owe us money” waiver is not an ABN. The form needs the specific service, a plain-language reason Medicare is expected to deny, and an estimated cost — a real number the patient can react to.

The patient then picks one of three options, and the choice has billing consequences. Option 1: they want the care and want Medicare billed for an official decision — the GA path, and it preserves their appeal rights. Option 2: they want the care but not the claim — no claim, so no modifier, and no appeal. Option 3: they decline. Staff must never choose for the patient, and the practice keeps the signed original.

Documentation that supports medical necessity

A modifier is a claim about your record. If the record can’t carry the claim, the modifier just tells a reviewer exactly where to look. For Medicare chiropractic, the load-bearing element is the demonstrated subluxation, and the standard is P.A.R.T.(CMS Medicare Benefit Policy Manual, chapter 15, §240.1.2): Pain/tenderness, Asymmetry/misalignment, Range-of-motion abnormality, and Tissue/tone changes. Documentation must support at least two of the four, and one of the two must be Asymmetry or Range of motion. Pain alone never gets there.

Beyond P.A.R.T., an AT claim needs a record showing care that is going somewhere: a functional baseline, a plan with goals, and visit-to-visit evidence of change. Its absence is what a plateau looks like on paper — which is why the shift to maintenance usually shows up in the chart before anyone says it out loud. The full anatomy of a defensible daily note, region by region, is in the SOAP notes and audits guide, including the cloned-note problem that makes reviewers assume the worst.

If you also bill therapy services

Practices delivering physical-therapy services under a qualifying arrangement — not under the chiropractic benefit — pick up a separate set of rules: the discipline modifier Medicare requires on therapy codes, the 8-minute rule on timed codes, and the annual per-patient therapy threshold above which continued therapy needs the KXmodifier attesting that your documentation supports medically necessary care. CMS updates that amount every year, and it counts therapy from every provider the patient has seen — so confirm the current figure against CMS rather than last year’s number. Spinal CMT is not a therapy service and never accrues toward it. The SOAP notes guide covers the KX and 8-minute mechanics in more detail.

What happens on audit

A records request is a reading exercise, and reviewers look for one mismatch: claims asserting active treatment on top of notes describing a patient who stopped changing. Three findings recur — AT on visits the chart shows as maintenance; manipulation billed without the exam findings that demonstrate a subluxation; and long stretches of identical notes. The remedy is repayment of the affected claims, and a post-payment review can reach back across a lot of visits — which is why drift is expensive in a way a single bad claim never is. The defense isn’t clever wording; it’s a chart where the phase is stated, the findings are specific to the day, and the claim never says anything the note doesn’t.

The habits that keep a practice clean

1. Decide the phase out loud, on a date. When a patient plateaus, write it in the assessment that day. Every modifier and ABN downstream depends on that one sentence.
2. Have the ABN conversation before the next visit, not during it.What Medicare covers, what it doesn’t, what continuing will cost. Patients handle this far better than we expect — what they don’t handle is a surprise balance.
3. Track the window, not just the signature. Know the date each ABN was signed and the date its course of treatment ends, and re-paper before it lapses.
4. Check your P.A.R.T. letters before you sign the note. Two of four, one of them A or R, on every Medicare manipulation visit.
5. Reconcile modifiers against phases monthly.Confirm each AT sits on an active-phase note and each GA inside a valid ABN window. Ten minutes a month finds drift while it’s still a correction rather than a disclosure.
6. Make the system do it.These are rules, and rules belong in software. If you’re evaluating practice software — or in the middle of switching systems — ask how it decides AT versus GA, where the ABN lives, and what stops a bare CMT line from reaching a payer. Vague answers are their own answer.

Medicare chiropractic billing earns its minefield reputation only when the chart and the claim are maintained from different assumptions. Keep them tied together — one documented phase, one modifier that follows it, one signed notice covering the dates it says it covers — and the whole thing collapses into a handful of habits you stop thinking about. To see how much of that a system can hold for you, the features tour walks through the note wizard, the modifier engine, and the billing spine behind them.