Under which circumstances would an add-on code NOT be used?

An add-on code is specifically designed to be used in conjunction with a primary procedure code to signify an additional service that enhances or complements the main procedure. An add-on code cannot be reported independently; it must always be linked to another primary procedure code that it accompanies.

The statement that an add-on code can stand alone is inherently incorrect, as this is contrary to the purpose of add-on codes themselves. If a code can independently represent a procedure without needing a primary procedure code, it is not classified as an add-on code but rather as a standalone procedure code. This illustrates why the correct choice highlights the requirement that add-on codes need to be associated with a primary code to be valid for billing and reporting.

In contrast, the assertion that a simple procedure might not use an add-on code depends on whether an additional service applies at all, but it does not negate the fundamental rule that add-on codes are designated to supplement primary procedures. In practice, the requirement for necessity and the relationship to primary codes primarily governs the correct application of add-on codes, rather than the complexity of the procedure alone.