How to Use the GP Modifier, KX Modifier, and GA Modifier for Accurate Therapy Billing

In healthcare, therapy isn’t just about helping patients recover—it’s also about keeping your clinic running smoothly. Billing properly is crucial to ensure you’re reimbursed accurately for the services you provide, especially when working with modifiers like the GP modifier, KX modifier, and GA modifier.

Whether you're managing claims for physical therapy, occupational therapy, or speech-language pathology, each modifier plays a unique role in the billing process. Incorrect or missing modifiers can lead to claim denials and delays, costing your clinic time, money, and resources.

Let’s dive into the specifics so you can streamline your billing process and stay on top of reimbursements.

Understanding modifier types in therapy billing

Billing modifiers help clarify the “how” and “why” behind the services you deliver. They’re essential when a CPT or HCPCS code alone doesn’t give the whole picture, especially for Medicare and other payers that demand precision.

There are two main types of modifiers used in therapy billing:

• CPT modifiers: These are two-digit numeric codes used to provide additional detail about the procedures listed under CPT codes. For example, modifier 59 is used to identify distinct procedural services that would otherwise be considered bundled, but it should only be applied when no more specific modifier is appropriate.

• Level II HCPCS modifiers: These are two-letter codes used to convey information related to Medicare billing, therapy discipline, or medical necessity. They often identify the type of provider or signal an exception to standard coverage limits. The GP, KX, and GA modifiers are most commonly used in therapy billing.

Why this matters: Many CPT codes are used across therapy disciplines, including physical therapy, occupational therapy, and speech-language pathology. Even though the services differ, the codes often look the same on paper. Without the correct modifier, payers can’t tell which discipline delivered the care or whether it meets coverage requirements. That can lead to denials, underpayments, or compliance issues.

As more clinics adopt RTM, getting the billing and modifier use correct is key to avoiding denials and delays. Want a full breakdown of RTM billing codes, documentation tips, and common mistakes to avoid? Download our free RTM Billing Guide PDF to simplify the process and get paid accurately.

Now, let’s examine the GP, KX, and GA modifiers more closely so you can apply them clearly and confidently.

What is the GP modifier?

The GP modifier tells Medicare, and many commercial payers, that a billed service was provided under an outpatient physical therapy plan of care. It’s a Level II HCPCS modifier that must be appended to most CPT codes commonly used in physical therapy.

Even though some CPT codes are commonly associated with therapy, they’re shared across multiple therapy disciplines (and can also be used by physicians/NPPs providing therapy under their license), making it difficult for payers to determine which type of provider delivered the service.

When to use the GP modifier

• Billing CPT codes that are delivered under a physical therapy plan of care (e.g., 97110, 97112, 97530)

Example

A patient receives therapeutic activities during a physical therapy session for post-operative knee rehabilitation. Because this intervention is billed as CPT code 97530, which is also used by occupational therapists, you append the GP modifier to show that the service was delivered under a physical therapy plan of care.

What is the KX modifier?

You've done everything right: followed the plan of care, helped your patient make progress, and documented each step along the way. However, now that the patient has reached Medicare's therapy threshold, and they still need skilled care. What do you do?

This is where the KX modifier comes in.

The KX modifier is your professional attestation that continued skilled therapy is medically necessary, even after the patient has exceeded Medicare's annual therapy threshold. It signals to Medicare (and many Medicare Advantage plans) that you're not just extending care.  Rather, the skilled care is clinically justified and the documentation is present to back it up.

When to use the KX modifier

• The patient has exceeded the Medicare annual threshold but still requires skilled care

• Continued skilled therapy is supported by documentation

• You’re billing Medicare or Medicare Advantage plans that follow CMS guidelines

If you know a patient is approaching the threshold, start documenting the clinical justification early, but wait to apply the modifier until needed. Premature or excessive use of the KX modifier can raise red flags during payer review.

Example

A patient completed physical therapy earlier in the year following a rotator cuff repair. Now, months later, they've returned after a fall and need further rehab for new shoulder weakness and balance deficits. After the first two therapy visits, the patient exceeds the threshold, and you append the KX modifier to your claim. The medical necessity for the skilled services is supported by the new diagnosis for which the patient is receiving therapy, and the associated plan of care and clinical documentation.

Keep in mind: If total therapy costs go beyond $3,000, the claim may be selected for targeted medical review.¹ Solid, defensible documentation will help ensure continued reimbursement and reduce audit risk.

What is the GA modifier?

The GA modifier is used when a service is not covered by Medicare or when the services are no longer medically necessary and the patient has signed an Advance Beneficiary Notice of Noncoverage (ABN).² This allows you to bill a secondary insurance or the patient directly, without violating CMS rules.

Not every therapy session is automatically covered by Medicare, especially when a patient has reached a functional plateau or wants to continue care for preventive or fitness reasons. That’s where the GA modifier becomes essential.

This modifier doesn’t guarantee payment, but it protects your clinic by showing that the patient was informed and agreed to financial responsibility in advance.

When to use the GA modifier

• You’ve provided the patient with a valid ABN explaining why the service is no longer medically necessary or the service is not covered by Medicare before the service was delivered

• The patient has signed an ABN, and you’re continuing to see the patient for fitness or wellness reasons

The GA modifier is used across healthcare for services like labs, imaging, DME, and more. But in rehab, it’s especially important when patients want to continue therapy that no longer meets Medicare’s definition of skilled, necessary care.

Example

A patient has completed six weeks of outpatient therapy following a total knee replacement. At this point, they’ve reached their functional goals and are no longer making measurable progress, but they’d like to continue coming to the clinic once a week to stay accountable and active.

You issue an ABN, explain the expected denial and bill the sessions with the GA modifier. This allows you to collect payment from the patient (or their secondary insurance) while staying compliant with Medicare rules.

5 tips to stay modifier-compliant

Even small modifier mistakes can lead to denied claims, delayed payments, or compliance flags. These tips can help your team apply modifiers correctly the first time.

1. Know your payer requirements. While Medicare sets the standard, commercial payers may have different rules for when and how modifiers should be applied.

2. Watch for overuse. Applying modifiers like KX or GA too routinely can trigger red flags for payers. Use them only when the criteria are met.

3. Keep your team trained. Annual billing updates, documentation training, and compliance refreshers can help prevent costly errors.

4. Audit yourself. Periodic internal reviews of modifier usage can catch patterns that put your clinic at risk and help fix them proactively.

5. Stay current. Modifier rules can change. Keep up with CMS updates and payer-specific guidance so you're not caught off guard.

Don’t let modifier confusion cost you 

Modifiers might seem minor, but they carry major financial and compliance weight. Used correctly, they speed up payments, reduce denials, and keep your clinic audit-ready. Used incorrectly, they trigger delays, lost revenue, and unnecessary rework.

Disclaimer: The information contained in this document does not, and is not intended to, constitute legal, billing, or regulatory advice or guidance. All information, content, and material is for general information purposes and independent review and/or counsel should be obtained before making any legal or billing decisions.

References

_{1. American Physical Therapy Association. (n.d.). Medicare payment thresholds for outpatient therapy services. APTA. https://www.apta.org/your-practice/payment/medicare-payment/coding-billing/therapy-cap}

_{2. Medicare Claims Processing Manual 100-04; Chapter 1; Section 60.4.1-Outpatient Billing with an ABN. https://www.cms.gov/regulations-and-guidance/guidance/manuals/downloads/clm104c01.pdf#page=172.}