Anti-Thrombin III

Function

Antithrombin III (ATIII) is a non-vitamin K-dependent glycoprotein that functions as a natural anticoagulant. It is primarily produced by the liver and acts by inactivating several enzymes in the coagulation cascade, most notably thrombin (Factor IIa) and Factor Xa. Its activity is significantly enhanced (by up to 1,000 times) in the presence of heparin, which is the physiological basis for heparin therapy in clinical medicine.

Why it is Ordered

Physicians order an ATIII test to investigate the cause of recurrent blood clots (thrombosis), such as Deep Vein Thrombosis (DVT) or Pulmonary Embolism (PE). It is also used to evaluate individuals who show resistance to heparin therapy—meaning they require unusually high doses of heparin to achieve adequate anticoagulation. Screening may also be performed in families with a history of hereditary antithrombin deficiency.

Associated Conditions

• Hereditary Deficiency: A genetic mutation leading to chronically low levels and high risk of venous thromboembolism.
• Acquired Deficiency: Often seen in liver disease (decreased production), nephrotic syndrome (loss through urine), or Disseminated Intravascular Coagulation (DIC) (excessive consumption during widespread clotting).
• Hypercoagulability: Low levels directly correlate with a 'pro-thrombotic' state, where the blood clots too easily.