ABN Billing 2026 (CMS-R-131, GA, GX, GY, GZ)

Get the Signature Before the Service or Absorb the Denial

By Yagnesh Dave  ·  October 2, 2026  ·  12 min read

A practice sends 30 Medicare patients a month for a lab test that Medicare is likely to deny on frequency. Each test costs $85. None of the patients signed an Advance Beneficiary Notice. Medicare denies all 30, and because there's no ABN the practice can't bill any of them. That's $2,550 a month and $30,600 a year, from a form that takes two minutes to sign.

ABN billing is simple on paper and messy at the front desk. The notice has one job: it tells the patient, before the service, that Medicare may not pay and that the patient could owe the bill. Without it, a medical necessity denial is almost always the provider's loss.

Here's when you need the ABN, how to fill it out, what GA, GX, GY and GZ mean on the claim, and how not to hand a patient a refund claim you could have avoided.

When ABN Billing Applies: Medicare's Limitation on Liability

Section 1879 of the Social Security Act protects a patient from paying for services Medicare denies as not reasonable and necessary, unless the patient knew. It also protects the provider when the provider didn't know and couldn't reasonably have been expected to know. Once you know Medicare probably won't pay, you've lost that protection unless the patient signs a valid notice.

The ABN is the form that moves the risk. CMS form CMS-R-131 is the standard version for Original Medicare physicians, providers and suppliers. Use it when you expect a denial because the service isn't reasonable and necessary for this patient, for example when frequency limits, diagnosis restrictions or an LCD coverage rule make a denial likely.

You don't use it for Medicare Advantage, which has its own notice rules, and you don't use it for a service Medicare never covers by statute, such as routine physicals or most cosmetic services. For those, a voluntary notice is optional and uses a different modifier, covered below.

GA, GX, GY and GZ: What Each Modifier Tells Medicare

The modifier is how the claim reports what happened with the ABN. Get it wrong and the claim processes under the wrong liability rule.

The remittance tells you whether it worked. With a valid ABN and GA, the denial comes back with group code PR, patient responsibility. Without one, a medical necessity denial arrives with CO, contractual obligation, and the patient can't be balance billed.

The common mistake is putting GA on a claim where no signed ABN exists, hoping to shift the liability. Medicare contractors compare the modifier to the record when they review the claim, and a GA with no ABN behind it looks like a false statement.

How to Complete the ABN Form Correctly

CMS says an ABN is only valid if every required field is filled in clearly before the service. The form has four parts that matter.

The notifier, patient and identification number

Fill in your name, the patient's name and the identification number (optional on the form, but useful for matching later). Don't leave the patient's name blank on a pre-printed copy.

Column D: the service or item

Name the specific service. "Lab tests" is too vague. "Vitamin D 25-hydroxy, once more than the covered frequency" works.

Column E: the reason Medicare may not pay

Give a plain reason: "Medicare doesn't usually pay for this test more than once a year," or "Medicare doesn't cover this service for your diagnosis." Write something specific and true.

Column F: the estimated cost

Give a good-faith estimate. If the estimate is far off, the patient's agreement may not hold. A range is acceptable as long as it's reasonable.

The three patient options

• Option 1: The patient wants the service and wants Medicare billed. You may collect payment now, but you owe the patient a prompt refund if Medicare pays or finds you liable. The patient keeps appeal rights.
• Option 2: The patient wants the service and doesn't want Medicare billed. The patient pays, and has no appeal.
• Option 3: The patient doesn't want the service. Nothing is billed and the patient owes nothing.

The patient picks the option, not the staff. Pre-checking Option 2 is the kind of thing that invalidates the whole form.

When an ABN Is Not Valid

Manual Chapter 30 lists the situations where CMS won't treat a notice as proof the patient knew. Billing teams get burned by a few of them over and over.

• Blanket notices. Giving every patient an ABN at check-in isn't acceptable, because it's routine and isn't tied to a real expectation of denial.
• Emergencies and duress. A notice handed to a patient in a medical emergency or under great pressure is invalid.
• A notice that's just a warning. "Medicare may not pay" with no specific service and reason doesn't qualify.
• A notice more than a year old. An ABN delivered more than one year before the service isn't acceptable evidence.
• An unreadable or misleading notice. Illegible forms, or forms where the staff coerced or misled the patient, fail the standard.

An ABN that's invalid is treated as if you never gave one. That means a GA claim denied for medical necessity can still land back on you.

Where ABN Revenue Is Lost

Most losses come from workflow, not from the form. Here are the patterns billing teams see over and over.

• Orders placed outside the visit. Standing lab orders are drawn at a draw station that never saw the ABN.
• The ordering office issues, the lab bills. If a physician gives the ABN and a lab bills the test, the lab needs a copy of the signed form, as CMS requires the notifier to pass it along.
• Expired forms. A patient signed an ABN 14 months ago for monthly services, and nobody reissued it.
• No cost estimate. The form was signed with column F blank.
• Wrong modifier. GZ used when an ABN was actually signed, or GA used when it wasn't.

Put numbers on it. If 5% of your Medicare Part B claims draw an expected medical necessity denial and you bill 1,000 Part B claims a month at an average of $120, that's $6,000 a month of denied value. With valid ABNs the patient or Medicare picks up most of it. Without them, you absorb nearly all of it.

ABN Audit and Medical Review Exposure

ABNs rarely get their own audit, but they decide who pays when a claim is denied after review. And medical necessity is one of the biggest sources of improper payments in Medicare.

CMS's fiscal year 2025 improper payment data puts the Part B provider rate at 8.44%, or $9.62 billion. Across Medicare fee-for-service, about 15.3% of improper payments were medical necessity errors and roughly 53% were insufficient documentation. Both categories are where an ABN either protects the practice or fails to.

Medicare Administrative Contractors use Targeted Probe and Educate to find providers with high error rates. A review typically runs up to three rounds, and a provider that stays noncompliant after three rounds is referred to CMS for further action. If your ABN process is weak, a TPE review of a service with expected denials is where it shows.

Keep copies for five years from the date care is completed, unless state law sets a longer period. When a reviewer asks for proof the patient knew, a retained, complete, signed form is the whole answer.

Frequently Asked Questions About ABN Billing

When do I need an ABN for Original Medicare?

When you expect Medicare to deny a service as not reasonable and necessary and the patient could be held responsible. You give the notice before you furnish the service. You don't need one for services Medicare never covers by law, though you can give a voluntary notice.

What's the difference between GA and GZ?

GA means you issued an ABN and the patient signed it. GZ means you expected a denial for medical necessity but didn't get a signed ABN, which means you can't bill the patient if Medicare denies.

How long is an ABN valid?

Up to one year for repetitive or continuing services, as long as the service, the reason for denial and the cost estimate haven't changed. A notice delivered more than a year before the service isn't valid evidence of knowledge.

Can I give every patient an ABN just in case?

No. CMS doesn't accept routine or blanket notices. An ABN has to name a specific service, a specific reason Medicare may deny it and a cost estimate for that patient.