Headline: CPT code 78120 covers a single-sample radiolabeled red cell volume determination, a nuclear medicine technique to quantify a patient’s red cell volume. Lead: CPT code 78120 represents a diagnostic procedure in which a measured quantity of radiolabeled red blood cells is administered and a single post-mixing blood sample is analyzed to calculate red cell volume — a targeted test used in select hematology and volume assessment contexts. National significance: The procedure is clinically relevant for evaluating blood volume disorders, anemia characterization, and pre-procedural hematologic assessment, and it has implications for resource use in outpatient nuclear medicine and hospital laboratories. Key payers covered: Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. What readers will learn: This publication explains what CPT code 78120 represents, its clinical purpose, typical sites of service, and the payer landscape addressed in the analysis. It provides benchmarks and coding context where available, highlights common billing modifiers and operational considerations, and summarizes implications for billing workflows. It also identifies where input data are missing. Scope: Content is national in focus and intended to inform coding, billing, and clinical stakeholders about the definition and practical context of CPT code 78120.