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Introduction ⬇

AT III inhibits the activity of activated factors XII, XI, IX, and X as well as factor II. AT III is the main physiologic inhibitor of activated factor X, on which it appears to exert its most critical effect. AT III is a “heparin cofactor.” Heparin interacts with AT III and thrombin, increasing the rate of thrombin neutralization (inhibition) but decreasing the total quantity of thrombin inhibited.

This test detects a decreased level of antithrombin that is indicative of thrombotic tendency. Only the test of functional activity gives a direct clue to thrombotic tendency. In some families, several members may have a combination of recurrent thromboembolism and reduced plasma antithrombin (30%–60%). A significant number of patients with mesenteric venous thrombosis have AT III deficiency. It has been recommended that patients with such thrombotic disease be screened for AT III levels to identify those patients who may benefit from coumarin anticoagulant prophylaxis rather than heparin therapy.

Normal Findings ⬆ ⬇

Functional Assay

Infants (1–30 days): 26%–61% or 0.26–0.61 (premature); 44%–76% or 0.44–0.76 (full term)

Adults and infants older than 6 months: 80%–120% or 0.80–1.20

Immunologic Assay

Adults and infants older than 6 months: 17–30 mg/dL or 170–300 mg/L

Procedure ⬆ ⬇

  1. Draw a venous blood sample (5 mL) using a tube with sodium citrate. Mix gently.

  2. Use the two-tube method. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered.

  3. Place the sample on ice and transport to the laboratory immediately.

Clinical Implications ⬆ ⬇

  1. Increased AT III values are associated with:

    1. Acute hepatitis

    2. Kidney transplantation

    3. Inflammation, patients with increased ESR

    4. Menstruation

    5. Use of warfarin (Coumadin) anticoagulant

    6. Hyperglobulinemia

  2. Decreased AT III values are associated with:

    1. Congenital deficiency (hereditary)

    2. Liver transplantation and partial liver removal, cirrhosis, nephrotic syndrome, liver failure

    3. DIC, fibrinolytic disorders (not diagnostically useful)

    4. Acute MI

    5. Active thrombotic disease (VTE), thrombophlebitis

    6. Carcinoma, trauma, severe inflammations

    7. Heparin failure (low levels of AT III exhibit heparin resistance)

    8. Protein-wasting diseases

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain test purpose and procedure.

  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 thrombotic tendency.

  2. If patient has decreased levels of AT III, coumarin anticoagulant would be used as a prophylaxis.

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

Interfering Factors ⬆

  1. Antithrombin decreases after 3 days of heparin therapy.

  2. Use of oral contraceptives interferes with the test (decreased values).

  3. Results are unreliable in the last trimester of pregnancy and in the early postpartum period.

  4. Decreased after surgery, prolonged bed rest.

  5. Decreased in L-asparaginase therapy.