CPT code 85303 represents a laboratory assay for measuring protein C activity in patient plasma. This test is used to assess the functional status of protein C, an anticoagulant protein with clinical relevance for patients evaluated for thrombophilia, unexplained thrombosis, or coagulopathy. Nationally, accurate reporting of 85303 supports clinical decision-making, appropriate use of anticoagulant therapies, and proper laboratory service reimbursement.

Key payers addressed include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise overview of clinical context and testing indications, typical sites of service, and commonly used billing modifiers. The publication also outlines payer coverage considerations and coding guidance relevant to laboratory and hospital billing workflows.

This summary prepares clinicians, laboratory billing staff, and health policy analysts to understand where 85303 fits within diagnostic testing for coagulation disorders, what sites perform the service, and which major national payers commonly cover such laboratory assays. Data not available in the input is noted where applicable.