Medicare beneficiaries may receive a Medicare Summary Notice or an Explanation of Benefits (if enrolled in a private plan) that has codes that make little sense unless you are a billing professional. While these codes are usually explained in the footnotes of the notices, understanding how Medicare billing and coding works can help a beneficiary avoid denials or overcharges. A Medicare GP modifier is an example of a billing code that beneficiaries may see on their notices.

Coding System

The Medicare GP modifier refers to a Medicare billing code under the current Healthcare Common Procedure Coding System. This coding system is an industry standard for billing Medicare. These codes are designed and implemented by the Centers for Medicare & Medicaid Services, which is the federal regulating agency that oversees Medicare.

GP Modifier

According to the Centers for Medicare and Medicaid Services, a GP modifier means that “Services [are] delivered under an outpatient physical therapy plan of care.” This means that the service or item received was a part of a preexisting plan of care for physical therapy created by Medicare doctors and physical therapists. It also means that the service was performed in an outpatient setting. Put another way, the patient did not need to be admitted to a hospital to obtain the service. In order for physical therapy to be covered by Medicare, a plan of care is required.

What Is a Modifier?

Sometimes a code alone is not sufficient enough to explain how the item should be charged. Variables affect how much the patient will be charged, whether the service is under any restrictions or if there is anything else required before this claim can be paid out by Medicare. A GP Modifier will accompany a code that more explicitly explains what the service was. For example, if a patient received in-home hydrotherapy as a part of his physical therapy plan of care, the code for in-home physical therapy (G0151) would be accompanied by the letters "GP." The full code would be G0151GP.

Code Levels

What the code that the GP modifier accompanies looks like depends on the code level. Codes are separated into three levels. Level I codes adhere to the American Medical Association’s Current Procedural Terminology and consists of numbers. Most services are found under Level I. Level II codes are both numbers and letters and are used to bill non-physician services. Level III are Medicaid codes. GP modifiers usually accompany Level II codes as physical therapy typically falls under this category.


The coding system was created in 1978 in order to standardize coding for Medicare and Medicaid. The codes are specific to the item or service being billed. However, until the Health Insurance Portability and Accountability Act of 1996 (HIPAA), it was only used voluntarily. Now HIPAA makes its use mandatory.

The Medicare Improvements for Patients and Providers Act of 2008 called for improvements and streamlining of the codes, which was implemented and carried out by the Centers for Medicare and Medicaid Services.