HCPCS Level II code Q5151 designates a 2 mg injection of eculizumab-aagh (epysqli), a biosimilar to the complement inhibitor eculizumab. This code captures administration of a therapeutic biologic used in conditions where complement inhibition is indicated and matters nationally as biosimilar biologics expand treatment access and influence payer coverage and hospital infusion workflows. Key payers included in the national analysis are Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will learn how Q5151 is used to represent dosing units for a biosimilar eculizumab product, the typical sites of service such as outpatient infusion centers and physician offices, and the clinical context for complement-inhibitor therapy. The publication provides benchmarks and payer coverage perspectives, summarizes relevant policy updates affecting biosimilar biologic billing and coding, and outlines implications for service-line operations and revenue cycle processes. Data not available in the input will be noted where applicable.