In orthopedics, laterality is everything. Modifier RT (Right side) and Modifier LT (Left side) are mandatory for procedures performed on paired organs or limbs (eyes, ears, arms, legs, knees, hips).
When to Use RT and LT
If a code descriptor is unilateral (meaning it describes treating one side), you MUST append RT or LT. Example: 27447-RT for a right total knee replacement.
RT/LT vs Modifier 50
If the surgeon performs the procedure on BOTH sides during the same operative session, you generally do NOT use RT and LT on two separate lines (unless specifically instructed by a commercial payer). For Medicare, you use a single line with Modifier 50 (Bilateral Procedure).
NCCI Edits and Add-on Codes
Do not append RT or LT to add-on codes or codes that inherently describe "bilateral" in their CPT definition. Doing so will confuse the claims system and result in a denial.