Antithrombin testing is primarily ordered, along with other tests for hypercoagulable disorders, to investigate to investigate the cause of recurrent venous thrombosis. The antithrombin activity test is performed before the antigen test, to evaluate whether the total amount of functional antithrombin is normal. Activity will be decreased with both type 1 and type 2 antithrombin deficiencies, so this test can be used as an initial screen for both. If the antithrombin activity is low, then the antithrombin antigen test is performed to determine the quantity of antithrombin present. If a deficiency is detected, both antithrombin tests are repeated at a later date to confirm test findings. Antithrombin testing may be used to evaluate patients not responding as expected to heparin.

Antithrombin activity testing is ordered, along with other tests for hypercoagulable disorders (such as Protein C and Protein S, and Lupus anticoagulant), when a patient has been experiencing recurrent venous thrombosis. Antithrombin should be measured after a blood clot has been treated and resolved as both the presence of the clot, and the therapy used to treat it, will affect antithrombin results.

Antithrombin testing may be ordered when a patient is not responding as expected to heparin anticoagulation - when unusually high doses of heparin are necessary to achieve the desired level of anticoagulation.

Decreased antithrombin activity and decreased quantities of antithrombin antigen suggest a type 1 antithrombin deficiency. In this case, the activity is decreased because there is less antithrombin available to participate in clotting regulation. Reduced antithrombin activity and normal levels of antithrombin antigen suggest a type 2 antithrombin deficiency. This means that there is sufficient antithrombin, but it is not functioning as it should.

If the antithrombin activity is normal, then the antithrombin antigen test is usually not performed. In this case, the antithrombin is functioning adequately and the recurrent thrombotic episodes being investigated are likely due to another cause.

Increased levels of antithrombin are not usually considered a problem.  They may be seen with:

• Acute hepatitis/cholestasis
• Kidney transplant
• Vitamin K deficiency
• Warfarin (coumadin) anticoagulation therapy (sometimes)

 Testing for antithrombin deficiency is not recommended unless the patient’s condition is stable. Temporarily or chronically decreased antithrombin levels may be seen with conditions that affect its consumption or production, such as:

• DIC (Disseminated intravascular coagulation), an acute or chronic condition characterized by the consumption of clotting factors. An affected patient may bleed and/or thrombose. 
• DVT (deep vein thrombosis – a blood clot usually in a deep leg vein)
• Liver disease
• Nephrotic syndrome
• Protein-losing condition
• Pulmonary embolism
• Neonates during the first few days of life
• Estrogen therapy

If a patient with an antithrombin deficiency also has other coagulation risks, such as a Protein C or S deficiency, a Factor V Leiden mutation, or oral contraceptive use, then she may be at a significantly elevated risk of developing a blood clot.

Antithrombin deficiency can increase the risk of recurrent miscarriage.

Patients with antithrombin deficiency may need to take preventative anticoagulation prior to medical or surgical procedures.

Antithrombin concentrates may be used to temporarily correct acute or chronic antithrombin deficiencies.