Modifier –50 (Bilateral Procedure) is appended to a CPT code when an identical procedure is performed on paired anatomical structures during the same operative session. The AMA states to add modifier 50 to the appropriate five‑digit code for bilateral procedures unless otherwise identified in the listings. CMS guidance (Medicare Claims Processing Manual, Ch. 23, §20.6.2) specifies that when modifier 50 is used, RT and LT modifiers must not be reported, and the service is submitted as a single line item with one unit of service. AAPC guidance aligns with this: use modifier 50 for diagnostic, radiology, or surgical procedures performed on both sides (eyes, ears, extremities, paired organs), but do not append it when the CPT descriptor already indicates “bilateral” or “unilateral or bilateral,” and do not use it with add‑on codes.

Modifier 50 affects reimbursement according to the CMS bilateral surgery indicator assigned to the CPT code in the National Physician Fee Schedule. For codes with indicator “1,” billing with modifier 50 typically triggers payment at 150% of the single‑side fee schedule (i.e., 100% + 50%). For codes with indicator “3” (often diagnostic or radiology), payment is generally 200% (two full single‑side amounts). Indicators “0” or “2” mean modifier 50 should not be used; in some cases a separate bilateral code exists or bilateral payment is handled differently. CMS transmittals and payer guidance explain that for codes where a unilateral descriptor and an existing bilateral code both exist, reimbursement rules (including lower‑of calculations) may apply to avoid overpayment.

Practical billing rules from CMS and coding educators: submit one claim line for the CPT code with modifier 50 and one unit of service; do not report RT/LT modifiers on the same line. Examples include billing 27447‑50 for bilateral total knee arthroplasty and 20605‑50 for aspiration/injection of both elbows when the CMS indicator supports bilateral billing (both examples reflect indicator=1 and application of the 150% adjustment per Medicare contractor guidance). Payers may apply their own bilateral payment rules consistent with the CMS indicators, so always confirm the specific code’s bilateral surgery indicator and payer policy before submission.

Because payer variation exists, review the National Physician Fee Schedule bilateral surgery indicator for each CPT code and follow CMS and payer instructions: use modifier 50 only when the code and payer permit it, submit a single line with one unit, and expect reimbursement adjustments according to the indicator (e.g., 150% for indicator 1, usually 200% for indicator 3).