HELP! Medicare G Modifiers: When to use which one?!

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I work in a public health agency and our contract with Medicare is as a immunization roster biller only (we can only bill/receive payment for Influenza and Pneumococcal vaccines). Since we are a public health agency, we do not turn away a patient with Medicare if they want something other than those two vaccines (yes, we advise that they can go to their PCP for most). Anyways,
we try to obtain ABN's for every patient, but due to the different programs within our clinic, some slip through without receiving an ABN.
Medicare has modifiers GA, GX, GY, and GZ to use to notify if an ABN was provided to the patient or not, but I am SO confused as to which one to use. Mainly it's the technical wording within the definition of each modifier that I struggle with (i.e. statutorily excluded vs. not being a Medicare covered benefit).

My question: Is there SOMEONE who would be willing to help me understand this better so I can bill these the correct way?!?! Thank you so much in advance!


True Blue
Modesto, CA
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This link is a great resource for understanding the process and modifiers:

· GA
(ABN was issued and signed bythe beneficiary as required by Medicare. Beneficiary liable.)

· GX
(ABN was issued and signed bythe beneficiary as voluntary. Beneficiary liable.)

· GY
(Indicates that the service isnot a benefit of Medicare in any definition. Can be billed in combination with GX if patient signed an ABN. Beneficiary is liable.)

· GZ
(Service is expected to bedenied and an ABN was not issued. Beneficiary not liable)
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