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Introduction ⬇

Protein C, a vitamin K–dependent protein that prevents thrombosis, is produced in the liver and circulates in the plasma. It functions as an anticoagulant by inactivating factors V and VIII. Protein C is also a profibrinolytic agent (i.e., it enhances fibrinolysis). The protein C mechanism therefore functions to prevent extension of intravascular thrombi. This test is used for evaluation of patients suspected of having congenital protein C deficiency. Resistance to protein C is caused by an inherited defect in the factor V gene (factor V Leiden) and causes significant risk for thrombosis. It is the underlying defect in up to 60% of patients with unexplained thrombosis and is the most common cause of pathologic thrombosis. If functional protein C is abnormal, a protein C resistance test should be performed.

This test evaluates patients with severe thrombosis and those with an increased risk for or predisposition to thrombosis. Patients with partial protein C or partial protein S deficiency (heterozygotes) may experience venous thrombotic episodes, usually in early adult years. There may be DVTs, episodes of thrombophlebitis or pulmonary emboli (or both), and manifestations of a hypercoagulable state. Patients who are heterozygous may have type I protein C deficiency, with decreased protein C antigen, or type II deficiency, with normal protein C antigen levels but decreased functional activity.

Normal Findings ⬆ ⬇

Qualitative: 70%–150% or 0.70–1.50 of increased functional activity

Quantitative: 60%–125% or 0.60–1.25 of normal PC antigen

Procedure ⬆ ⬇

  1. Draw a 5-mL venous plasma blood sample into a tube with sodium citrate (light blue–topped tube). The two-tube method is used. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

  2. Place the specimen on ice.

Clinical Implications ⬆ ⬇

  1. Decreased protein C is associated with:

    1. Severe thrombotic complications in the neonatal period (neonatal purpura fulminans)

    2. Increased risk for venous thrombotic episodes

    3. Warfarin (Coumadin)-induced skin necroses (pathognomonic for protein C deficiency)

    4. DIC, especially when it occurs with cancer (presumably owing to consumption by cofactor thrombin–thrombomodulin catalyst activities)

    5. Thrombophlebitis and pulmonary embolism, especially in early adult years

    6. Other acquired causes of protein C deficiency include:

      1. Liver disease

      2. Acute respiratory distress syndrome

      3. L-Asparaginase therapy

      4. Malignancies

      5. Vitamin K deficiency

  2. A deficiency of protein C may also be congenital (35%–58%).

Clinical Alert

Patients with homozygous protein C deficiency have absent or almost-absent protein C antigen and usually succumb in infancy with clinical presentation of purpura fulminans neonatalis, including lower extremity skin ecchymoses, anemia, fever, and shock

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and procedure. Patient should be fasting.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings; explain the need for possible follow-up testing and treatment. Monitor for thrombosis. In the case of a protein C deficiency, educate the patient concerning the symptoms and implications of the disease. The risk factors include obesity, oral contraceptives, varicose veins, infection, trauma, surgery, pregnancy, immobility, and heart failure.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Decreased protein C is found in the postoperative state.

  2. Pregnancy or use of oral contraceptives decreases protein C.

  3. A transient drop in protein C occurs with a high loading dose of warfarin (Coumadin).

  4. Protein C decreases with age.

  5. High doses of heparin decrease protein C.

  6. Lipemic serum may interfere with the assay.