Here’s the CPT® manual’s definition of this commonly-used modifier:
“Distinct Procedural Service: Under certain circumstances, it may be necessary to indicate that a procedure or service was distinct or independent from other non-E/M services performed on the same day. Modifier -59 is used to identify procedures/services, other than E/M services, that are not normally reported together, but are appropriate under the circumstances.
“Documentation must support a different session, different procedure or surgery, different site or organ system, separate incision/excision, separate lesion, or separate injury (or area of injury in extensive injuries) not ordinarily encountered or performed on the same day by the same individual. However, when another already established modifier is appropriate, it should be used rather than modifier -59. Only if no more descriptive modifier is available, and the use of modifier -59 best explains the circumstances, should modifier -59 be used.”