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Levels of Antithrombin (formerly known as Antithrombin III) are used in the investigation of patients with a history of unexplained or recurrent thrombosis, a suspected or established diagnosis of DIC, and apparent "resistance" to heparin therapy.

Decreased levels are associated with an increased risk of thrombosis. Acquired Antithrombin deficiency may be due to:

  1. Decreased hepatic synthesis; e.g. in cirrhosis, chronic hepatitis, congestive heart failure, fatty liver of pregnancy, newborn babies.
  2. Increased consumption by activation of coagulation factors; e.g. in DIC, pulmonary embolism, trauma, deep vein thrombosis.
  3. Increased renal or intestinal loss; e.g. in nephrotic syndrome and protein losing enteropathy.

Hereditary Antithrombin deficiency is inherited in an autosomal dominant manner. Affected individuals have Antithrombin levels between 25 and 50% of normal. The most common clinical manifestations are idiopathic or recurrent deep vein thrombosis of arms and legs, pulmonary embolism, or mesenteric vein thrombosis, occurring initially at a young age, often precipitated by pregnancy or surgery.

Drugs can also affect Antithrombin levels. Decreased levels are associated with heparin therapy, estrogen, and L-asparaginase. Coumadin increases the Antithrombin level.

Reference range is 80 - 130%.

Specimen requirement is one light blue top (sodium citrate) tube of blood. The specimen should be delivered to the laboratory within 2 hours. If not possible, it should be centrifuged and the plasma frozen at -20 C.

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