85303

Protein C, Antigen Assay

Protein C is a critical vitamin K-dependent plasma glycoprotein synthesized in the liver that serves as a major regulator of the blood coagulation cascade. It acts as a natural anticoagulant by circulating in an inactive zymogen form until it is converted into activated protein C (APC) by the thrombin-thrombomodulin complex on the vascular endothelial surface. Once activated, APC, in the presence of its cofactor Protein S, proteolytically inactivates coagulation factors Va and VIIIa. These factors are essential for the conversion of prothrombin to thrombin; thus, their inactivation effectively limits further clot formation. CPT code 85303 refers to the quantitative laboratory measurement of the Protein C antigen level in a patient's plasma. This test is typically performed using immunological methods such as Enzyme-Linked Immunosorbent Assay (ELISA) or latex-enhanced immunoturbidimetric assays, which utilize specific antibodies to bind to the Protein C molecule regardless of its functional state. This procedure is distinct from the Protein C activity assay (CPT 85302), which measures the enzyme's actual inhibitory function. Measurement of the antigen is essential for identifying and classifying Protein C deficiencies into two primary categories. Type I deficiency is characterized by a quantitative decrease in both the total amount of Protein C (antigen) and its activity levels. Type II deficiency, or qualitative deficiency, is characterized by a normal concentration of the protein (antigen) but a functional defect that reduces its anticoagulant activity. Testing for Protein C antigen is indicated in patients with a personal or strong family history of venous thromboembolism (VTE), particularly when thrombosis occurs at a young age, in unusual sites, or is recurrent. It is also used to evaluate neonates presenting with purpura fulminans, a life-threatening condition associated with homozygous Protein C deficiency.

Clinical Indications

  • Evaluation of patients with unexplained or recurrent venous thromboembolism (VTE)
  • Investigation of thrombophilic tendencies in patients under age 50
  • Screening for hereditary Protein C deficiency in individuals with a positive family history
  • Assessment of neonatal purpura fulminans or severe neonatal thrombotic disease
  • Classification of Protein C deficiency as Type I (quantitative) or Type II (qualitative)
  • Investigation of warfarin-induced skin necrosis
  • Evaluation of patients with thrombosis at unusual anatomical sites (e.g., mesenteric or cerebral veins)

Procedure Steps

  1. Obtain a peripheral venous blood sample from the patient using a sodium citrate (light blue top) anticoagulant tube.
  2. Ensure proper filling of the collection tube to maintain the correct 9:1 blood-to-anticoagulant ratio.
  3. Centrifuge the blood sample immediately to separate the plasma from cellular components (double-spin may be required to achieve platelet-poor plasma).
  4. Aliquot the plasma and, if not tested immediately, freeze at -20 degrees Celsius or lower to maintain protein stability.
  5. Prepare the immunological assay (e.g., ELISA plate coated with anti-Protein C antibodies or latex reagent for turbidimetry).
  6. Add patient plasma and standards to the assay system, allowing for the binding of Protein C antigen to the specific antibodies.
  7. Incubate the mixture for a specified duration according to the manufacturer's protocol.
  8. Add a detection conjugate (e.g., enzyme-labeled antibody) followed by a substrate to produce a measurable signal.
  9. Quantify the signal (colorimetric or light scattering) using a spectrophotometer or automated analyzer.
  10. Calculate the concentration of Protein C antigen by comparing the patient results against a standard reference curve and report as a percentage of normal.

Coding Guidelines

  • CPT 85303 is used specifically for the Protein C antigen (quantitative) measurement.
  • Do not confuse 85303 with CPT 85302, which is for Protein C activity (functional assay).
  • If both antigen (85303) and activity (85302) are performed to differentiate Type I and Type II deficiency, both codes may be reported.
  • Testing should ideally be performed when the patient is not in an acute thrombotic state, as Protein C levels are often consumed during active clotting, leading to false-positive deficiency results.
  • Testing should be delayed until the patient has been off Vitamin K antagonist therapy (e.g., Warfarin) for at least 2 weeks, as these medications decrease Protein C production.
  • If the test is performed as part of a larger coagulation or thrombophilia panel, ensure each component is billed with its specific CPT code unless a specific panel code applies.

Associated ICD-10 Codes