Thrombophilia: How To Test? How To Manage?

By Julie Hambleton, MD

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Antithrombin

AT inactivates thrombin and factors Xa, IXa, XIa, and XIIa. The anticoagulant activity toward activated factors II (thrombin) and X is enhanced by the presence of heparin. The inheritance pattern of AT deficiency is autosomal dominant. Symptomatic AT deficiency in the general population is not rare, with an estimated frequency between 1:2,000 and 1:5,000, but it is still an uncommon abnormality. However, the majority of individuals with AT deficiency will develop VTE by their third decade of life. In unselected patients with a history of VTE (first episode of unprovoked DVT), the frequency of AT deficiency is 1.1%. In selected patients with a history of recurrent VTE and/or familial history, the frequency is approximately 2.4%. The majority of affected individuals are heterozygotes, with AT levels between 40 and 70% of normal. There are three types of AT deficiency among individuals with the disorder, and the vast majority of cases can be diagnosed by determination of the functional AT level, a widely available clinical test.

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