Final answer:
The GX modifier is used in medical billing to indicate that a service is not expected to be covered by Medicare and that the patient has been informed via an ABN, agreeing to pay out of pocket.
Step-by-step explanation:
In the context of healthcare billing and Medicare, a GX modifier is used to indicate that a service has been provided to a patient, and the service is expected to be denied as not reasonable and necessary.
This modifier is attached to a claim to acknowledge that an Advance Beneficiary Notice (ABN) was issued to the patient. An ABN is a written notice from Medicare, given to the patient before services are provided, when Medicare is expected not to pay for some or all of the services. The GX modifier alerts the payer that the patient has been informed and agrees to be financially responsible for the charges if Medicare denies payment.