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Congenital antithrombin III deficiency is a genetic disorder that causes the blood to clot more than normal.

Antithrombin III is a protein in the blood that naturally blocks blood clots from forming. Congenital antithrombin III deficiency is an inherited disease. It occurs when a person receives one abnormal copy of a gene from a parent with the disease.

The abnormal gene leads to low levels of antithrombin III. These low levels of antithrombin III can cause abnormal blood clots (thrombi) that may damage organs.

Often, patients with this condition will have a blood clot at a young age and will have a family member who has also experienced a blood clotting episode.

Patients will usually have symptoms of a blood clot, including:

• Coughing up blood
• Fainting
• Shortness of breath and pain when taking deep breaths
• Swelling of one leg

A physical examination may show:

• A fast heart rate
• A swollen foot or ankle

The diagnosis is made by checking for low levels of antithrombin III in the patient's blood.

A blood clot is treated with blood thinning medications or anticoagulants. How long you need to take these medications depends on how serious the blood clot was and other factors. Discuss this with your health care provider.

Most patients have a good outcome if they stay on anticoagulant medications.

Blood clots can cause death, especially if they are in the lungs.

See your health care provider if you have symptoms of this condition.

Because this is an inherited condition, there is no way to prevent it. However, once a patient is diagnosed with antithrombin III deficiency, all close family members should be screened.

Deficiency - antithrombin III - congenital; Antithrombin III deficiency - congenital

Schafer A. Thrombotic disorders: Hypercoagulable states. In: Goldman L, Ausiello D. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier;2007:chap 182.

Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.