CPT modifier used for procedure performed on infants less than 4 kg. Procedures performed on neonates and infants up to a present body weight of 4 kg may involve significantly increased complexity and physician work commonly associated with these patients. This circumstance may be reported by adding the modifier -63 to the procedure number. Unless otherwise designated, this modifier may only be appended to procedures/services listed in the 20000-69999 code series. Modifier -63 should not be appended to any codes listed in the Evaluation and Management Services, Anesthesia, Radiology, Pathology/Laboratory, or Medicine sections.