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Antithrombin III Antigen Blood Test

An antithrombin III antigen blood test is used to help investigate the cause of recurrent inappropriate blood clotting or to help diagnose an antithrombin deficiency.

Test Code: 5158

CPT Code: 85301

Also Known As: functional antithrombin III; AT III


Fixed Rate Time Nephelometry

Specimen Type: Blood


Fasting for 10-12 hours required. Avoid anabolic steroids, Gemfibrozil, Warfarin (Coumadin®), heparin therapy, asparaginase, estrogens, gestodene and oral contraceptives for 3 days prior to specimen collection. Check with your physician before stopping any medications.

Test Results:

8-12 days. May take longer based on weather, holiday or lab delays.


Antithrombin testing is primarily ordered, along with other tests for excessive clotting disorders, to investigate the cause of recurrent blood clot formation (thrombosis).  Previously referred to as "antithrombin III", antithrombin antigen is an inhibitor of several coagulation factors. Patients with low concentrations of Antithrombin Antigen may have a hereditary or acquired prothrombotic state. The antigenic test differentiates a Type I from Type II deficiency.

Antithrombin III testing is also used when a person is not responding as expected to heparin anticoagulation and when unusually high doses of heparin are necessary to achieve the desired level of anticoagulation.

Examples of disorders and conditions associated with increased blood clotting include Deep venous thrombosis, Phlebitis (vein inflammation) and Pulmonary embolus (blood clot traveling to the lung).

If your antithrombin level is lower than normal, you likely have an antithrombin deficiency. This puts you at an increased risk of developing DVT and other conditions, including:

  • phlebitis, or inflammation of the veins
  • thrombophlebitis, or inflammation of the veins that causes a clot to form
  • a pulmonary embolus, or a blood clot in the lung
  • a heart attack if the blood clot travels to arteries of the heart
  • a stroke if the blood clot travels to arteries of the brain

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