1. In Current Procedural Terminology (CPT) coding, a two-character (numeric or alphanumeric) add-on indicator placed after the usual procedure code number to indicate circumstances in which a procedure as performed differs in some way from that described by its usual five-digit code (see Box M-1 ). Also called code modifier . 2. In HCPCS Level II coding, a one- or two-digit alpha or alphanumeric number placed after the usual Level I or II code that helps further describe a procedure or service. Also see Healthcare Common Procedure Coding System (HCPCS) modifiers .Box M-1MODIFIER27372-51 (typed on one line)