Add-On Code

A CPT code that can only be billed in conjunction with a primary procedure code, not on its own. Identified by a + symbol in the CPT book. Common examples: 99417 (additional E/M time, billed only with 99205, 99215, or 99483), 99458 (additional 20 minutes RPM monitoring, only with 99457). Submitting an add-on code without its primary triggers immediate denial.