Deep vein thrombosis (DVT) is a type of venous thrombosis involving the formation of a blood clot in a deep vein, most commonly in the legs or pelvis. A minority of DVTs occur in the arms. Symptoms can include pain, swelling, redness, and enlarged veins in the affected area, but some DVTs have no symptoms. The most common life-threatening concern with DVT is the potential for a clot to embolize (detach from the veins), travel as an embolus through the right side of the heart, and become lodged in a pulmonary artery that supplies blood to the lungs. This is called a pulmonary embolism (PE). DVT and PE comprise the cardiovascular disease of venous thromboembolism (VTE). About two-thirds of VTE manifests as DVT only, with one-third manifesting as PE with or without DVT. The most frequent long-term DVT complication is post-thrombotic syndrome, which can cause pain, swelling, a sensation of heaviness, itching, and in severe cases, ulcers. Recurrent VTE occurs in about 30% of those in the ten years following an initial VTE.

The mechanism behind DVT formation typically involves some combination of decreased blood flow, increased tendency to clot, changes to the blood vessel wall, and inflammation. Risk factors include recent surgery, older age, active cancer, obesity, infection, inflammatory diseases, antiphospholipid syndrome, personal history and family history of VTE, trauma, injuries, lack of movement, hormonal birth control, pregnancy, and the period following birth. VTE has a strong genetic component, accounting for approximately 50 to 60% of the variability in VTE rates. Genetic factors include non-O blood type, deficiencies of antithrombin, protein C, and protein S and the mutations of factor V Leiden and prothrombin G20210A. In total, dozens of genetic risk factors have been identified.

People suspected of having DVT can be assessed using a prediction rule such as the Wells score. A D-dimer test can also be used to assist with excluding the diagnosis or to signal a need for further testing. Diagnosis is most commonly confirmed by ultrasound of the suspected veins. VTE becomes much more common with age. The condition is rare in children, but occurs in almost 1% of those ≥ age 85 annually. Asian, Asian-American, Native American, and Hispanic individuals have a lower VTE risk than Whites or Blacks. Populations in Asia have VTE rates at 15 to 20% of what is seen in Western countries.

Using blood thinners is the standard treatment. Typical medications include rivaroxaban, apixaban, and warfarin. Beginning warfarin treatment requires an additional non-oral anticoagulant, often injections of heparin. Prevention of VTE for the general population includes avoiding obesity and maintaining an active lifestyle. Preventive efforts following low-risk surgery include early and frequent walking. Riskier surgeries generally prevent VTE with a blood thinner or aspirin combined with intermittent pneumatic compression.