Deep vein thrombosis (DVT) is the formation of a blood clot ("thrombus") in a deep vein. Deep vein thrombosis commonly affects the leg veins (such as the femoral vein or the popliteal vein) or the deep veins of the pelvis. Occasionally the veins of the arm are affected (such as in Paget-Schrötter disease). A DVT can occur without symptoms, but in many cases the affected extremity will be painful, swollen, red, and warm, and the superficial veins may be engorged. The most serious complication of a DVT is that the clot could dislodge and travel to the lungs, which is called a pulmonary embolism (PE). DVT and PE are the two manifestations of the disease venous thromboembolism (VTE). DVT is a medical emergency, so, all limb swellings, however trivial, should be regarded as a DVT until proven otherwise. Untreated lower extremity DVT has a 3% PE-related mortality rate. Deaths associated with upper extremity DVT are extremely rare. A late complication of DVT is the post-thrombotic syndrome, which can manifest itself as edema, pain or discomfort and skin problems.

According to Virchow's triad, venous thrombosis occurs due to three factors: decreased flow rate of the blood (venous stasis), damage or activation of the blood vessel wall and an increased tendency of the blood to clot (hypercoagulability). Several medical conditions can lead to DVT, such as compression of the veins, physical trauma, cancer, infections, certain inflammatory diseases and specific conditions such as stroke, heart failure or nephrotic syndrome. There are several factors which can increase a person's risk for DVT, including older age (the strongest risk factor), surgery, hospitalization, immobilization (such as when orthopedic casts are used or during long-haul flights), heavy smoking, obesity, certain drugs (such as estrogen or erythropoietin) and inborn tendencies to form clots known as thrombophilia (for example, in carriers of factor V Leiden). Women have an increased risk during pregnancy and in the postnatal period, due to increased estrogen levels.

The most commonly used tests for the diagnosis of DVT are a blood test called D-dimers and doppler ultrasound of the affected veins. Sometimes, further testing is required to find the cause of the DVT. In specific cases, an attempt can be made to break down the clot (using thrombolytic agents). To prevent further accrual and formation of new clots with a risk of pulmonary embolism, anticoagulation (blood thinners) is advised (if not possible, an inferior vena cava filter may be used). Prevention of DVT is advised in many medical and surgical inpatients using anticoagulants, graduated compression stockings (also known as thromboembolic deterrent stockings) or intermittent pneumatic compression (IPC) devices.

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