Definition of Congenital antithrombin III deficiency:

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

Causes, incidence, and risk factors:

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. In people with this condition, a blood clot may occur without the common risk factors of pregnancy, lack of movement (due to surgery or trauma), or use of oral birth control pills.