Why use a modifier?
•    to indicate that a service or procedure that has been performed has been altered by some specific circumstance but not changed in its definition or code.
•    may use a modifier to report —
* a service or procedure has either a professional or a technical component
*  a service or procedure was provided more than once
*  a service or procedure has been increased or reduced
* only part of a service was performed
*  unusual events occurred
*  a bilateral procedure was performed

Some modifiers affect reimbursement and others are for documentation purposes.

CPT Modifiers:

21 Prolonged E/M Services
Add to E/M code number
22 Unusual Procedural Services
Service greater than that usually required for the procedure
23 Unusual anesthesia
Procedure that usually requires no anesthesia or local anesthesia must be done under general anesthesia.
24 Unrelated E/M Service by Same Physician During a Postoperative Period
E/M service performed during a postoperative period for a reason unrelated to the original procedure
25
Significant, Separately identifiable E/M Service by the Same Physician on the Same Day of the Procedure or Other Service
E/M services required above and beyond the other service provided or beyond the usual pre and postoperative care associated with the proce-dure performed. Different diagnoses not required.
ws26 Professional Component When physician component reported separately from technical component
32 Mandated Services
Services related to mandated consultation or related services
47 Anesthesia by Surgeon Regional or general anesthesia provided by the surgeon (does not include local anesthesia)
50 Bilateral Procedure
Bilateral procedures performed at the same operative session
51 Multiple Procedures Multiple procedures, other than E/M*, performed at the same session by the same provider. Report the primary procedure as listed and add -51 to the additional code(s).
52 Reduced Services
Procedure partially reduced or eliminated at the physician’s discretion
53 Discontinued Procedure Physician elects to terminate procedure that was started but discontinued. Not used for elective cancellation of a procedure prior to patient’s anesthesia induction and/or surgical preparation in the operating suite.
54 Surgical Care Only One physician performs surgical procedure and another the pre and/or postoperative care. Add -54 to the surgical procedure code.
55 Postoperative Management Only Used when one physician performed preoperative care and another performed the surgical procedure.
56 Preoperative Management Only Used when one physician performed preoperative care and another performed the surgical procedure.
57
Decision for Surgery
An E/M service that resulted in decision to perform surgery.
58 Staged or Related Procedure or Service by the Same Physician During the Postoperative Period
Physician should indicate that procedure was 1) planned prospectively (staged); b) more extensive than original procedure; or 3) for therapy following a surgical procedure. Not used to report treatment of problem requiring return to operating room.
59    Distinct Procedural Service
Service is distinct or independent from other services performed on same day. Identifies procedures not normally reported together.
62    Two Surgeons
When two surgeons work together as co-surgeons performing distinct parts of a single reportable procedure, each adds -62 to the surgery code, and any associated add-on code(s) for that procedure as long as both surgeons continue to work together as primary surgeons.
63    Procedure Performed on Infants less than 4 kg
To allow for increased complexity of procedures performed on neonates and infants up to 4 kg. May be appended to codes in 20000 – 69999 range only; not used with E/M, Anesthesia, Radiology, Pathology/Labora tory, or Medicine
66    Surgical Team
Highly complex procedures may require concomitant services of several physicians. Each physician adds -66 to surgery code.
76    Repeat Procedure by Same Physician
Procedure or service repeated subsequent to the original.
77    Repeat Procedure by Another Physician
Basic procedure done by another physician had to be repeated.
78    Return to the Operating Room for a Related Procedure During the Postoperative Period
Another procedure was performed during the postoperative period of the original procedure that was related to the first.
79    Unrelated Procedure or Service by the Same Physician During the Postoperative Period
Performance of procedure during postoperative period was unrelated to the original procedure.
80    Assistant Surgeon
Use -80 modifier to identify surgical assistant services.
81 Minimum Assistant Surgeon
Use -81 modifier to identify minimal surgical assistant services.
82 Assistant Surgeon (when qualified resident surgeon not available) Unavailability of qualified resident surgeon is a prerequisite for use of modifier -82.
90 Reference (Outside) Laboratory Laboratory procedures performed by a party other than the treating or reporting physician.
91 Repeat Clinical Diagnostic Laboratory Test Repeat same lab test on same day to obtain multiple test results.
99 Multiple Modifiers Two or more modifiers may be necessary to completely delineate a service.
NOTE: Anesthesia Physical Status Modifiers P1 – P6 are included in Appendix A, as well as the Anesthsia Guidelines.