This was covered in the August 2013 Q&A Webinar. Join the
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GA is "Waiver of liability statement issued as required by payer policy, individual case." This means that an ABN is on file and allows the provider to bill the patient if the service is not covered by Medicare.
GX is "Notice of liability issued, voluntary under payer policy" This means that an ABN is on file, the provider knows Medicare does not pay for the service, but is looking for a denial from Medicare so that they can bill the secondary insurance.