Modifier 79 is appended to a procedure to indicate that the same surgeon took the patient back to the operating room during the global surgical period for an unrelated problem.  The second procedure must be unrelated to the original procedure.  See modifier 78 for return trips to the OR that are related, and modifier 58 for return trips to the OR that were planned or staged.

The second procedure must be performed by the same surgeon during the global post op period.  The reason for the surgery must be unrelated to the first surgery.   A new global period starts when the physician uses this modifier.

On Feb 13, 2009, CMS issued a transmittal about this topic.  It is attached as a resource.  It notes that using modifier 79 bypassed carrier edits, which prevented payments during the global period.  However, the OIG found that some providers were using it to bypass these edits when it was not appropriate. 

Medicare's rules for billing during the global period are in the Medicare Claims Processing Manual, Publication 100-04, Chapter 12, Section 40.