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Antithrombin III (Activity)

Antithrombin III Activity

Antithrombin III Activity testing assesses your most important natural anticoagulant. Including this in a thrombophilia screen provides critical information about inherited or acquired clotting risk.

Doctor's Assessment Included

Antithrombin III (Activity): what this test measures

This test measures the functional anticoagulant activity of antithrombin III, expressed as a percentage of normal. It assesses how effectively your antithrombin inhibits clotting factors.

Antithrombin III (Activity): why this value matters

Antithrombin deficiency is the strongest inherited thrombophilia risk factor. Identifying low activity helps guide decisions about anticoagulation during high-risk situations such as surgery, pregnancy, or prolonged immobility.

Antithrombin III (Activity): when is testing worthwhile?

Testing could be indicated for unexplained venous thrombosis, family history of thrombophilia, heparin resistance, or as part of a comprehensive thrombophilia screen. Do not test during acute thrombosis or heparin therapy as results may be unreliable.

Antithrombin III (Activity): symptoms of a high or low value

Low Levels

Low antithrombin activity may be associated with increased risk of venous thromboembolism including deep vein thrombosis (DVT) and pulmonary embolism (PE). Symptoms may include leg swelling, pain, redness, sudden shortness of breath, or chest pain.

High Levels

Elevated antithrombin activity is generally not clinically significant and does not typically require medical intervention.

Antithrombin III (Activity): lifestyle and this value

If antithrombin deficiency is confirmed, reduce modifiable thrombosis risk factors: maintain a healthy weight, stay active, avoid smoking, stay hydrated, and use compression stockings during long travel. Discuss anticoagulation plans with your healthcare provider before surgery or pregnancy.

Antithrombin III (Activity): frequently asked questions

What is antithrombin deficiency?

A condition where your natural anticoagulant protein is insufficient, increasing clotting risk. It can be inherited or acquired from liver disease, nephrotic syndrome, or DIC.

Why might heparin not work if antithrombin is low?

Heparin works by enhancing antithrombin activity. If antithrombin levels are very low, heparin cannot exert its full anticoagulant effect, a condition known as heparin resistance.

Can antithrombin levels change over time?

Yes. Levels can be temporarily reduced by acute thrombosis, liver disease, pregnancy, oestrogen therapy, and DIC. Repeat testing when stable is important for accurate assessment.