Deep vein thrombosis, usually abbreviated DVT, is a blood clot that forms in a deep vein, most commonly in the leg. The term "deep" distinguishes these veins from superficial veins located closer to the skin. DVT commonly involves the deep veins of the calf, thigh or pelvis, although thrombosis can occur in deep veins of the arm or other anatomical regions. A DVT partially or completely obstructs venous blood flow and can cause local inflammation and swelling. Its major systemic danger is that part of the thrombus can detach, travel through the venous circulation and lodge in the pulmonary arteries, producing a pulmonary embolism. DVT and PE are therefore commonly grouped together as venous thromboembolism.
DVT develops when conditions favour thrombosis within the venous circulation. Virchow's triad provides a useful conceptual framework: venous stasis, vascular or endothelial activation or injury, and increased coagulability. Reduced movement is especially important because skeletal-muscle contractions normally help return venous blood from the legs towards the heart. When someone is immobile for prolonged periods, blood can pool in the deep veins, creating conditions favourable to clot formation. Surgery, hospitalisation, prolonged bed rest and long journeys can therefore increase risk. The presence of one risk factor does not make DVT inevitable, and some people develop DVT without an identifiable trigger.
DVT can occur after surgery because several pro-thrombotic mechanisms operate simultaneously. Surgical tissue injury activates coagulation, inflammation can increase clotting activity, and postoperative immobility reduces venous flow. A plaster cast or other immobilising device can similarly reduce calf-muscle pumping. Hospitalisation itself is associated with increased risk, especially when mobility is substantially reduced. Long-distance travel lasting more than four hours can increase risk, particularly when combined with additional factors such as previous DVT, recent surgery, pregnancy or thrombophilia. The absolute risk for an individual traveller is not determined solely by journey duration; personal risk factors are important. Regular movement and appropriate preventive measures can reduce risk in susceptible circumstances.
Several medical and physiological conditions increase the likelihood of DVT. These include cancer, heart failure, inflammatory bowel disease, vasculitis, obesity, smoking, older age, varicose veins and inherited or acquired thrombophilia. Pregnancy and the first six weeks after childbirth are recognised higher-risk periods. Oestrogen-containing contraception and HRT can increase venous thrombotic risk. A previous DVT is also important because recurrence can occur. These factors operate through different mechanisms: some increase coagulation, some alter venous flow, some affect the vessel wall and some combine several mechanisms. NHS guidance specifically lists these conditions and circumstances among recognised risk factors.
DVT is not always symptomatic. Some people have a clot that produces few or no noticeable local symptoms, which means absence of swelling or pain does not exclude the condition. When symptoms do occur, they typically affect one leg and can include throbbing or cramping pain, swelling, warmth, red or darkened skin and prominent or swollen veins. The calf or thigh is a common location for discomfort. Upper-extremity DVT can instead cause swelling and discomfort in an arm. Pelvic vein thrombosis may produce lower abdominal discomfort. The clinical presentation is therefore variable, and symptoms alone cannot reliably confirm or exclude DVT.
The diagnosis of DVT usually requires objective assessment because many other conditions can cause leg pain or swelling. Duplex or compression ultrasound is commonly used because it can show abnormal venous flow and whether the vein can be compressed normally. Clinicians may first estimate the probability of DVT using the clinical history and examination and may use a D-dimer blood test in appropriate circumstances. A positive D-dimer does not specifically prove DVT because fibrin breakdown increases in many conditions. If DVT is suspected, timely assessment is important because untreated thrombosis can extend or embolise. NHS guidance indicates that people suspected of having DVT should generally be assessed promptly and may be referred for ultrasound within 24 hours.
Treatment usually involves anticoagulation. Medicines such as direct oral anticoagulants or heparin-based treatments reduce the blood's ability to form and extend clots while the body's natural fibrinolytic mechanisms gradually break down the existing thrombus. The duration of treatment depends on whether the DVT was provoked, unprovoked, recurrent or associated with an ongoing risk factor, as well as the person's bleeding risk and other clinical circumstances. Anticoagulants do not make the blood literally "thin"; they interfere with specific components of the coagulation process. Compression, mobilisation and other supportive measures may also be used in selected circumstances. Severe or unusual cases may require specialist interventions.
The most serious immediate complication of DVT is pulmonary embolism. If part of a venous thrombus breaks away, it becomes an embolus and travels through the inferior or superior vena cava to the right side of the heart and then into the pulmonary arterial circulation. A sufficiently large embolus can obstruct pulmonary blood flow, impair gas exchange and place strain on the right ventricle. DVT can also have longer-term consequences such as post-thrombotic syndrome, which may involve chronic leg swelling, discomfort, heaviness and skin changes caused by damage to venous valves and persistent venous hypertension. A DVT should therefore be regarded as more than a temporary local leg problem.
