Antithrombin III (AT-III, AT-III Activity, Heparin Cofactor

Test Description


During hemostasis, a substance called thrombin stimulates the formation of fibrin
from fibrinogen. This fibrin then forms a stable clot at the site of injury. Any excess
amounts of clotting factors, which remain following hemostasis are inactivated by
fibrin inhibitors which prevent clotting from occurring when it is not needed. One
such substance is antithrombin III (AT-III).
AT-III is a naturally occurring protein immunoglobulin that is synthesized by the
liver. The action of AT-III is catalyzed by heparin. Its role is to inactivate thrombin
and other coagulation factors, thus inhibiting the coagulation process. The proper
balance between thrombin and AT-III allows for appropriate hemostasis to occur. If,
however, this balance is disrupted, problems can arise. For example, if there is a
congenital deficiency of AT-III, coagulation will not be inhibited at an adequate
level, resulting in a hypercoagulability state with a high risk of thrombosis.

THE EVIDENCE FOR PRACTICE

In women without a personal history of venous thromboembolism but with an underlying
antithrombin deficiency that is identified through screening, hormone replacement therapy
is not recommended.

Normal Values (Functional method)

                   Premature infant: 26–61%
                   Full-term infant: 44–76%
                   After 6 months: 80–120%

Possible Meanings of Abnormal Values

Increased                                                                                      Decreased        

Vitamin K deficiency                                                               Cirrhosis
                                                                                                 Congenital AT-III deficiency
                                                                                                 Deep vein thrombosis
                                                                                                 Disseminated intravascular coagulation                                                                                                     Hypercoagulation state
                                                                                                 Late pregnancy/early postpartum period
                                                                                                 Liver transplant
                                                                                                 Malnutrition
                                                                                                 Nephrotic syndrome
                                                                                                 Postoperative period
                                                                                                 Pulmonary embolism
                                                                                                 Septicemia

  Contributing Factors to Abnormal Values

• Hemolysis of the blood sample and presence of lipemia may alter test results.
• Drugs that may increase AT-III levels: anabolic steroids, androgens, progesteronecontaining
  oral contraceptives, warfarin.
• Drugs that may decrease AT-III levels: estrogen-containing oral contraceptives, fibrinolytics,
   heparin, L-asparaginase.

Interventions/Implications

Pretest

• Explain to the patient the purpose of the test and the need for a blood sample to be drawn.
• No fasting is required before the test.

Procedure

• A 7-mL blood sample is drawn in a light blue-top collection tube containing sodium citrate.
  The sample is mixed gently by inverting 3 to 4 times.
• Gloves are worn throughout the procedure.

Post test

• Apply pressure at venipuncture site. Apply dressing, periodically assessing for continued
   bleeding.
• Label the specimen, place it on ice, and transport it to the laboratory immediately.
• Report abnormal findings to the primary care provider.

Clinical alert                     

• In adults, AT-III levels of 50 to 75% indicate a moderate risk for thrombosis; levels

of <50% suggest a significant risk for thrombosis.