We are a plastic surgery practice whose surgeon often performs bilateral placement of tissue expanders in immediate breast reconstruction. He will also place acellular dermal tissue matrices at the time of the expansion (CPT code 15777, an add-on code). We typically bill 19357-50 and 15777-50. As the 15777 code is an add-on code, the bilateral procedures have NOT been reduced accordingly to multiple procedure guidelines when billed with the 19357 (even though 19357 second breast is reduced). One insurance is now trying to retract 1/2 payment on the second 15777 code. Should this second 15777 be reduced according to multiple procedure guidelines even though it is an add-on code?