Modifier 79 is used to indicate that the physician performed a surgical service that required a return trip to the OR for an unrelated problem during the global post op period.  Modifier 79 is appended to procedures. See the CPT® book for the complete definition.  It is appended when:

• A second surgery is provided to a patient by the same surgeon
• The surgery requires a return to the OR
• The patient is in a global period
• The service is unrelated to the initial surgery

Using modifier 79 restarts the global period. 

Show that the second procedure was not related to the first by the documentation (indication, etc) and the diagnosis code submitted on the claim form.

CMS recently instructed carriers/ MACs  to increase their review of claims with this modifier.