Each modifier is based on a specific aspect of the unbundled procedure – when the procedures took place, the anatomical locations where they were performed, who performed the services or what made the second service unusual:
1. XE (Separate encounter, a service that is distinct because it occurred during a separate encounter). 2. XS (Separate structure, a service that is distinct because it was performed on a separate organ/structure). 3. XP (Separate practitioner, a service that is distinct because it was performed by a different practitioner). 4. XU (Unusual non-overlapping service, the use of a service that is distinct because it does not overlap usual components of the main service).
You'll still be able to use modifier 59, but CMS notes you shouldn't use it when a more specific modifier is available. Your carrier may also require an X modifier for codes that have a high risk of incorrect billing.