Definition	Venous thromboembolism is any thromboembolic event occurring within the venous system, including deep venous thrombosis and pulmonary embolism. Deep venous thrombosis is a radiologically confirmed partial or total thrombotic occlusion of the deep venous system of the legs sufficient to produce symptoms of pain or swelling. Proximal deep venous thrombosis affects the veins above the knee (popliteal, superficial femoral, common femoral, and iliac veins). Isolated calf vein thrombosis is confined to the deep veins of the calf and does not affect the veins above the knee. Pulmonary embolism is radiologically confirmed partial or total thromboembolic occlusion of pulmonary arteries, sufficient to cause symptoms of breathlessness, chest pain, or both. Post-thrombotic syndrome is edema, ulceration, and impaired viability of the subcutaneous tissues of the leg occurring after deep venous thrombosis. Recurrence refers to symptomatic deterioration caused by a further (radiologically confirmed) thrombosis, after a previously confirmed thromboembolic event, where there had been an initial, partial, or total symptomatic improvement. Extension refers to a radiologically confirmed new, constant, symptomatic intraluminal filling defect extending from an existing thrombosis.
Incidence/Prevalence	We found no reliable study of the incidence/prevalence of deep venous thrombosis or pulmonary embolism in the United Kingdom. A prospective Scandinavian study found an annual incidence of 1.6 to 1.8 per 1,000 people in the general population.1,2 One postmortem study estimated that 600,000 people develop pulmonary embolism each year in the United States, of whom 60,000 die as a result.3
Etiology/Risk Factors	Risk factors for deep venous thrombosis include immobility, surgery (particularly orthopedic), malignancy, smoking, pregnancy, older age, and inherited or acquired prothrombotic clotting disorders.4 Evidence for these factors is mainly observational. The oral contraceptive pill is associated with death caused by venous thromboembolism (absolute risk increase with any combined oral contraception: 1 to 3 per million women per year).5 The principal cause of pulmonary embolism is a deep venous thrombosis.4
Prognosis	The annual recurrence rate of symptomatic calf vein thrombosis in people without recent surgery is more than 25 percent.6,7 Proximal extension develops in 40 to 50 percent of people with symptomatic calf vein thrombosis.8 Proximal deep venous thrombosis may cause fatal or nonfatal pulmonary embolism, recurrent venous thrombosis, and the post-thrombotic syndrome. One observational study published in 1946 found 20 percent mortality from pulmonary emboli in people in the hospital with untreated deep venous thrombosis.9 One non-systematic review of observational studies found that, in people after recent surgery who have an asymptomatic calf vein deep venous thrombosis, the rate of fatal pulmonary embolism was 13 to 15 percent.10 The incidence of other complications without treatment is not known. The risk of recurrent venous thrombosis and complications is increased by thrombotic risk factors.11
Clinical Aims	To reduce acute symptoms of deep venous thrombosis and to prevent morbidity and mortality associated with thrombus extension, the post-thrombotic syndrome, and pulmonary embolization; to reduce recurrence; and to minimize any adverse effects of treatment.
Clinical Outcomes	Rates of symptomatic recurrence, post-thrombotic syndrome, symptomatic pulmonary embolism, and death. Proxy outcomes include radiologic evidence of clot extension or pulmonary embolism.