The GA Modifier:
The GA modifier is the waiver of liability statement issued as required by payer policy, in an individual case.
What this really means: Medicare covers the service only under certain circumstances, and you expect Medicare to deny the service as not reasonable and necessary in this case. When you use this modifier you expect your practice to hold the patient financially responsible for the service if Medicare denies the claim, because you have a signed ABN. (Eli Research, Medicare Compliance & Reimbursement, 2011 pg. 156, Special Issue Vol. 27, No. 20, Kalpona Moitra, Editor)
The GX Modifier:
The GX Modifier is when the notice of liability is issued, voluntary under payer policy.
What this really means: Medicare never covers the service (it is statutorily excluded), but you got the patient to sign an ABN anyway. You will get an automatic denial from Medicare, and then you can submit the claim to a secondary payer. The patient is fully responsible for the charges.
The GZ Modifier:
The GZ modifier is used when an item or service is expected to be denied as not reasonable and necessary.
What this really means: the service is not medically necessary under Medicare rules based on the frequency or other coverage criteria such as payable diagnosis. The practice did not obtain an ABN, so the practice is responsible for the charges and cannot bill the patient OR the secondary insurance.