Final answer:
The modifier GA should be added to CPT/HCPCS codes on Medicare claims when an ABN has been signed, indicating the patient is aware they may be responsible for payment if Medicare doesn't cover the service.
Step-by-step explanation:
When an Advance Beneficiary Notice (ABN) has been signed on Medicare claims, it indicates that the service provided may not be covered by Medicare. In such cases, the modifier GA should be appended to the CPT/HCPCS codes on the claim forms. This modifier is an important indicator to Medicare that the patient has been informed and has agreed to pay for the services if Medicare does not cover them.