CPT code 85305 represents a laboratory diagnostic assay that quantifies total protein S levels in patient plasma. Protein S testing is clinically important for evaluating inherited or acquired thrombophilia and for guiding anticoagulation-related clinical decisions. Nationally, accurate coding and coverage determinations for protein S testing affect laboratory reimbursement, utilization monitoring, and access to targeted coagulation diagnostics.

Key payers in this analysis include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise clinical context for the assay, standard sites of service, and an overview of typical billing considerations. The publication also summarizes benchmarking approaches and payer coverage patterns where available, highlights relevant coding relationships, and outlines common modifiers and service-line implications for laboratories and hospital billing teams. Where payer-specific policy or reimbursement details are not provided in input data, the report indicates that such data are not available. The content is organized to help laboratory managers, billing professionals, and policy analysts understand how CPT code 85305 is used in clinical and administrative workflows and what documentation elements commonly accompany this diagnostic test.