Modifiers GX, GY and GZ: The Liability Decision Against GA

The Medicare liability modifiers are usually taught as four separate definitions, which is why they are so often mixed up. They are better understood as one decision with two axes. Ask why Medicare will not pay — because the service is not reasonable and necessary in this instance, or because it falls outside the statutory benefit altogether — and ask whether a notice was given to the beneficiary before the service. Answer both and the modifier is determined. There is no discretion left.

The consequential cell in that matrix is GZ. It describes exactly the same clinical and coverage situation as modifier GA — an expected medical-necessity denial — and differs only in that no Advance Beneficiary Notice was issued. That single difference moves the entire cost from the beneficiary to the provider, and it puts a marker on the claim that a contractor can count. This guide is about the whole matrix; the GA page covers the waiver mechanics in depth and is the anchor for the notice-was-given half.

The Decision, in Two Questions

Question one: why is coverage in doubt?

• Medical necessity. Medicare would ordinarily cover this item or service, but you expect it to be denied in this instance as not reasonable and necessary — a frequency limit, a Local Coverage Determination the patient does not meet, a documentation position you know will fail. This is the mandatory ABN territory. The modifier will be GA or GZ.
• Statutory exclusion. The item or service is excluded by statute, or does not meet the definition of any Medicare benefit. No coverage determination is going to change that. This is voluntary notice territory. The modifier will be GY, GX, or both.

Question two: was a notice issued?

• Medical necessity with a proper ABN → GA. Beneficiary liable.
• Medical necessity without an ABN → GZ. Provider liable.
• Statutory exclusion, no notice given (none is required) → GY. Beneficiary liable.
• Statutory exclusion, voluntary notice given → GX, which may be reported with GY.

The Matrix

Chapter 1 of the Medicare Claims Processing Manual (Publication 100-04), Section 60.4.2, tabulates these modifiers for institutional billing, and Section 60.5 supplies the remittance codes each denial produces.

Modifier	Why coverage is in doubt	Notice	Liability	How the line is submitted	Remittance
GA	Expected denial as not reasonable and necessary	Mandatory ABN issued	Beneficiary	As a covered charge; Medicare makes a payment determination	Depends on the determination
GZ	Expected denial as not reasonable and necessary	No ABN issued — and GZ cannot be used where one actually was	Provider	As non-covered; denied	Group Code CO, CARC 50, MSN 8.81
GY	Statutorily excluded, or does not meet the definition of any Medicare benefit	Not required	Beneficiary	As non-covered; denied	Group Code PR, CARC 96, RARC N425, MSN 16.10
GX	Statutorily excluded or not a Medicare benefit	Voluntary notice issued	Beneficiary	As non-covered; denied. May be reported with GY	As for the non-covered line

Read the Group Code column and the whole framework becomes concrete. CO means contractual obligation — the provider absorbs it and cannot bill the patient. PR means patient responsibility. Section 60.5 states the convention directly: when the beneficiary is liable, contractors use Group Code PR; when the provider is liable, Group Code CO. GZ produces CO. GY produces PR. That is the entire practical difference between the two, expressed in two letters on a remittance advice.

The covered-versus-non-covered mechanic

GA is the odd one out and it trips people up. Section 60.4.2’s entry for GA specifies that the line item must be submitted as covered, with Medicare making a determination for payment. GZ, GY and GX lines all go in as non-covered and are denied. The logic is that GA asks Medicare to adjudicate an ordinarily-covered service while flagging that the beneficiary has already accepted liability if the answer is no; submitting a GA line as non-covered defeats that. A corollary in the same section: GA and KB cannot be used with HCPCS code A9270 (non-covered item or service) precisely because those modifiers require covered charges, and A9270 is non-covered by definition. GY, by contrast, works with A9270 — the section notes that liability on an A9270 line rests with the provider unless a modifier such as GL or GY assigns it to the beneficiary.

On institutional outpatient claims, Section 60.4.1 adds the claim-level layer: ABN-related claims use occurrence code 32 with the date the beneficiary signed the notice, and never condition codes 20 or 21 (claims received with those are returned). Where a claim carries both ABN-related and unrelated services, occurrence code 32 still identifies the notice and only the GA lines are treated as ABN-related and must be covered charges. For entirely non-covered statutory-exclusion claims, Section 60.1.1 points instead to condition code 21, a claim-level code signifying that all charges on the claim are non-covered.

GZ vs. GA — The One That Costs Money

These two describe an identical situation. Same service, same expected denial, same reason. The only difference is whether someone handed the beneficiary a compliant notice before the service was furnished — and that difference decides who absorbs the cost.

Contractor guidance states the GZ position without euphemism: the provider expects a medical necessity denial but did not provide an ABN, and the line containing the GZ modifier is denied provider-liable. All charges are submitted as non-covered. If Medicare determines the service is not payable, the denial is a medical necessity denial and the message tells the beneficiary they are not responsible for payment.

Three things follow that are worth internalising:

1. The GA/GZ choice is made at the front desk, not in the billing office. By the time a coder sees the claim, the answer has already been determined by whether a proper notice exists. No amount of billing skill converts a GZ into a GA. Section 60.4.2 is explicit that GZ cannot be used where an ABN or HHABN was actually given — you cannot go the other way either.
2. GZ is an honest modifier, and using it is better than not using it. The alternative to a GZ is not a GA; it is an unmodified claim that misrepresents the situation. Contractor guidance notes a genuine operational benefit: where either the beneficiary or the provider requests a review, the modifier indicates that no ABN was given and can help complete the review more quickly. Section 60.1.3.1 also observes that GZ is often used on entirely non-covered claims precisely as the indicator that shows provider liability on all denied line items.
3. Volume is what draws attention. A GZ line is a self-report that the notice process did not happen. One is a miss. A pattern of them is a control failure that is visible in claims data without anyone requesting a chart, and it sits directly upstream of the question of whether services were being furnished with no realistic expectation of coverage.

Where GZ appears for reasons other than a missed ABN

Two places in the manual use GZ as the standing marker for provider liability, and neither is a billing error. Section 150.3.3 provides that in the expedited-review process, where a provider is not timely in giving information to the Quality Improvement Organization or does not give valid notice to the beneficiary, any services required in consequence are billed as non-covered line items using the GZ modifier, which indicates the provider is liable. And Section 60.4.2’s table describes GZ as available for optional use on demand bills not related to an ABN, by providers who want to acknowledge that they did not provide an ABN for a specific line.

GY vs. GZ — The Distinction Most Often Collapsed

Both produce a denied, non-covered line. They are not interchangeable, and the Group Code proves it: GY denies PR (patient responsibility); GZ denies CO (provider liability).

The axis is why Medicare will not pay. GY says the item is outside the statutory benefit — there is nothing to adjudicate, and no notice is required, because a beneficiary is presumed to know that Medicare does not cover services Congress excluded. GZ says the item is inside the benefit but is expected to fail the reasonable-and-necessary test, and that the notice which would have shifted liability was not given. Using GY on a medical-necessity situation asserts a statutory exclusion that does not exist; using GZ on a statutorily excluded item accepts liability the provider never had.

Contractor guidance for GY adds practical detail: append it when services are provided under a statutory exclusion, the claim will deny whether the modifier is present or not, it is not necessary to give the patient an ABN in these situations, and its function is to cause the claim to deny with the patient liable for the charges. Non-covered ambulance mileage reported on HCPCS A0888 is a named example. Incorrect uses named: appending GY to bundled procedures, and appending it to add-on codes.

GX — The Voluntary Notice

GX reports that a voluntary ABN was issued for something statutorily non-covered or outside any Medicare benefit definition. It exists because providers frequently — and sensibly — give patients written notice about services Medicare will never pay for, even though no notice is required. GX documents that courtesy on the claim.

The rules that matter:

• Append it only on lines submitted as non-covered charges.
• It may be reported in combination with GY, or on its own. Section 60.4.2 records this pairing explicitly, and contractor guidance repeats it.
• Do not append it where no ABN was issued. Like GA, GX asserts that a notice exists.
• Do not append it to items that are not statutorily excluded or that meet a defined benefit category. A voluntary notice does not convert a medical-necessity situation into a statutory one — if the doubt is about medical necessity, the notice needed to be a mandatory ABN and the modifier is GA.

GX is the low-risk member of the set. It neither shifts liability that would not otherwise fall on the beneficiary nor accepts liability the provider does not have. It records that the patient was told.

The Misuse That Triggers Denials and Audits

1. Stacking contradictory liability modifiers on one line. Section 60.4.2 closes with the warning that modifiers indicating provider liability cannot be used on entirely no-payment claims for which the beneficiary has liability, and that inappropriate use of these modifiers may result in entire claims being returned to providers. GA, GZ and GY express incompatible positions and do not belong on the same line.
2. GZ as a habit. Appending it routinely to insulate against denials, rather than fixing the notice process that produces GZ situations.
3. GY on a medical-necessity denial. Asserts a statutory exclusion that does not exist, and puts the cost on the beneficiary on a false basis.
4. GX where no notice was given, or GX on an item that is not statutorily excluded.
5. GA on a non-covered line. Mechanically self-defeating — GA requires covered charges.
6. Using any of them to move a research cost onto a patient. In the clinical-trial setting an ABN cannot lawfully shift a sponsor-owed research cost to a beneficiary; doing so compounds double-billing and False Claims Act exposure rather than avoiding it. See the clinical trial billing compliance checklist and the False Claims Act in billing.
7. Using an ABN on a bundled service. Section 60.4.3 states that an ABN may not shift liability where full payment is bundled into other payments, because collecting separately for a bundled item constitutes double billing. An ABN has to apply to all of a bundled service or none of it — so the whole bundle is billed as non-covered, or none of it is.

A Working Sequence

1. Establish why payment is in doubt: medical necessity, or statutory exclusion. Read the applicable Local Coverage Determination if the answer is not obvious — this is where the determination is actually made.
2. If medical necessity: was a compliant, item-specific, beneficiary-specific ABN issued before the service, with the beneficiary selecting an option? Yes → GA, on a covered line, with occurrence code 32 on an institutional claim. No → GZ, non-covered.
3. If statutory exclusion: was a voluntary notice given? Yes → GX, optionally with GY. No → GY alone.
4. Check that only one liability position appears on the line.
5. If the answer came out GZ, treat it as a process finding as well as a claim decision, and look at why the notice was not issued.

Where Jurisdiction Matters

The ABN form (CMS-R-131), the mandatory-versus-voluntary distinction, the prohibition on blanket notices and the four-modifier framework are national. What is not national is the coverage determination underneath — and that is what decides which branch of the matrix you are on. Local Coverage Determinations differ between contractors, so the same service can be routinely covered in one jurisdiction and require an ABN in another. Novitas, Noridian, First Coast Service Options, Palmetto GBA, WPS and CGS each maintain their own LCD sets, and each publishes its own modifier guidance with differing emphasis.

Two scope limits are worth stating plainly. The ABN and the G-series apply to fee-for-service Medicare; Medicare Advantage plans operate their own notice, denial and appeal frameworks, and a Part B ABN is not the right instrument there. And commercial payers use their own waiver-of-liability and patient-responsibility forms with their own timing and content rules; an ABN issued for a commercial claim generally shifts nothing. Where you cannot find guidance on point, the two questions still hold: is the doubt about medical necessity or statutory scope, and was a compliant notice given before the service.

Frequently Asked Questions

What is the difference between modifier GA and modifier GZ?

Whether an ABN was issued. Both describe a service expected to be denied as not reasonable and necessary. GA says a notice was given, so the beneficiary is liable and the line goes in as a covered charge for Medicare to adjudicate. GZ says no notice was given, so the provider is liable and the line goes in as non-covered and is denied.

What is the difference between GY and GZ?

Why Medicare will not pay, and therefore who pays. GY is for statutorily excluded items and denies with Group Code PR — patient responsibility. GZ is for expected medical-necessity denials with no ABN and denies with Group Code CO — provider liability.

Can GX and GY be used together?

Yes. Section 60.4.2 records that GX may be used in association with GY, and contractor guidance repeats it. GX documents that a voluntary notice was issued for a statutorily excluded item; GY identifies the exclusion.

Should I use GZ or just leave the modifier off?

Use it. GZ describes the situation accurately and, according to contractor guidance, can speed a review by showing on the face of the claim that no ABN was given. An unmodified claim in the same circumstances simply misdescribes the service. The right response to a GZ is to fix the notice process, not to hide the marker.

Do I need an ABN for a statutorily excluded service?

No. Contractor guidance for GY states that it is not necessary to provide a patient with an ABN in statutory-exclusion situations. Many providers give a voluntary notice anyway as a matter of practice, and GX records that.

Can GA and GY appear on the same line?

No. They express contradictory positions — GA asks Medicare to adjudicate an ordinarily-covered service, GY states the item is outside the statutory benefit. The manual warns that inappropriate use of these modifiers may result in entire claims being returned to providers.

What denial code does a GZ line produce?

Contractors deny services submitted with modifier GZ using Group Code CO, CARC 50, and Medicare Summary Notice message 8.81. A GY line denies with Group Code PR, CARC 96, RARC N425 and MSN 16.10. Remittance code assignments are periodically revised — check the current code sets.

What is modifier GK?

GK identifies a reasonable and necessary item or service associated with a GA or GZ modifier — typically the covered companion to a non-covered line in an upgrade scenario. It is not an alternative to GA or GZ; it sits alongside one.

Related CASRAI Resources

• Modifier 32: mandated services — a common route into this matrix, and one that usually lands on GY rather than GA
• Modifier GA: shifting liability with an ABN on file — the anchor page for the notice-was-given half of this matrix
• Advance Beneficiary Notice (ABN) mechanics — what makes a notice valid in the first place
• Modifier KX: what you are attesting to — the opposite posture on the same coverage question
• Medicare coverage determinations: NCDs and LCDs — where the branch of the matrix is actually decided
• Clinical trial billing compliance checklist
• The False Claims Act in billing
• OIG Self-Disclosure Protocol for billing overpayments
• Modifier 91: repeat clinical diagnostic laboratory test — frequency limits are a common ABN trigger
• Modifiers 62 and 66: co-surgeons and surgical teams

Sources: CMS Medicare Claims Processing Manual, Publication 100-04, Chapter 1, Sections 60.1.1, 60.1.3.1, 60.4.1 (outpatient billing with an ABN, occurrence code 32), 60.4.2 (line-item modifiers related to reporting of non-covered charges), 60.4.3 (liability considerations for bundled services), 60.5 (coding that results from processing non-covered charges) and 150.3.3; Noridian Healthcare Solutions modifier GX guidance (last updated 22 September 2025) and modifier GY and GZ guidance (both last updated 12 May 2025). Remittance advice code assignments are periodically revised — verify against current code sets. The coverage determination underlying any of these modifiers is jurisdiction-specific; check the LCD that applies to your claim. General reference material, not billing advice for a specific claim.