Antithrombin III deficiency - congenital Definition Antithrombin III deficiency is a genetic disorder that causes the blood to clot abnormally. Alternative NamesDeficiency - antithrombin III - congenital; Antithrombin III deficiency - congenital Causes Antithrombin III is a protein in the blood that blocks blood clots from forming. Congenital antithrombin III deficiency is a genetic 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 |