I got the following info from
http://www.capturebilling.com/commonly-used-medicare-modifiers-ga-gx-gy-gz/GX Modifier:
Notice of Liability Issued, Voluntary Under Payer Policy.
Report this modifier only to indicate that a voluntary ABN was issued for services that are not covered.
Medicare will automatically reject claims that have the –GX modifier applied to any covered charges.
Modifier –GX can be combined with modifiers –GY and –TS (follow up service) but will be rejected if submitted with the following modifiers: EY, GA, GL, GZ, KB, QL, TQ
Additional information on the –GX modifier can be found at:
http://www.cms.gov/mlnmattersarticles/downloads/MM6563.pdfGY Modifier:
Notice of Liability Not Issued, Not Required Under Payer Policy. This modifier is used to obtain a denial on a non covered service. Use this modifier to notify Medicare that you know this service is excluded.