Add-on procedures are commonly carried out in addition to a primary procedure. The codes representing add-on procedures are identified in the American Medical Association (AMA) CPT book with a "+" symbol and are listed in Appendix D of the CPT manual. Add-on codes are always performed in addition to a primary procedure. They should never be reported as standalone codes. An add-on code billed without the accompanying code for the primary procedure may be contested. They are exempt from multiple procedure reduction rules and should not be billed with modifier 51.