CPT code 85301 represents a laboratory immunoassay for antithrombin III antigen in plasma, used to detect and quantify antithrombin III protein levels relevant to coagulation and thrombophilia evaluation. Nationally, this assay supports diagnosis and monitoring of bleeding and clotting disorders and informs clinical decisions about anticoagulation management and hereditary deficiency assessment. Coverage and payment for this laboratory service affect hospital and independent laboratory workflows and laboratory testing utilization patterns.

Key payers covered in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise overview of clinical context for the assay, typical sites of service, common modifiers and billing considerations, and gaps where input data were not provided. The publication outlines typical coding usage, expected service line placement for laboratory claims, and the role of this antigen assay in diagnostic pathways.

This brief provides national-level context rather than state-specific policy. Data not available in the input are identified explicitly where applicable, and clinical background is summarized to support coding and billing clarity around CPT code 85301.