Instructions and Appendix A explain the use of what type of codes when a specific procedure or service is altered?

Modifiers are a critical component in medical coding, used to provide additional information about a procedure or service that has been altered in some way without changing its definition or code. When a service is performed that differs from the standard protocol — perhaps due to a unique circumstance, complexity, or delivery method — modifiers are applied to explain these variations.

For instance, if a procedure took longer than usual or was performed in a different manner than initially planned, a modifier would accurately convey this to the payers for reimbursement purposes. This helps ensure that the healthcare provider is compensated appropriately for the nuances of the service provided. In contrast, unbundled codes, bundled codes, and add-on codes are different types of coding practices used primarily for the grouping of services or to indicate additional services, but they do not specifically address the alteration of procedures in the same way that modifiers do.