The formation of a thrombus (BLOOD CLOT) in one part of the circulatory system from which a portion becomes detached and lodges in another blood vessel, partially or completely obstructing the blood flow (an EMBOLISM). Most commonly a thrombus is formed in the veins of the leg – DEEP VEIN THROMBOSIS (DVT) –and the embolism lodges in the pulmonary (lung) circulation (also referred to as VTE – venous thromboembolism). PULMONARY EMBOLISM is a potentially fatal condition and requires urgent anticoagulant treatment (see ANTICOAGULANTS) and sometimes surgery. Extended periods lying in bed or prolonged sitting in a confined position such as a car or aeroplane can cause DVT; venous thromboses in the legs may occur after surgery and preventive anticoagulant treatment with HEPARIN and warfarin is often used. Similar treatment is needed if a thrombus develops. STREPTOKINASE is also used to treat thromboembolism.