Acute deep venous thrombosis (DVT) is the formation of a stationary blood clot in one or more deep veins of the leg. The calf veins are the most common sites of a thrombus (clot), but 40% of DVTs occur in the femoral (thigh) and iliac (hip) veins behind the valves (which prevent backflow of blood). Symptoms of this obstruction are pain, swelling, warmth, and redness in the affected limb.

The three main factors in the development of DVT are abnormalities in blood flow, abnormalities of the blood itself, and injury to the blood vessel wall. Imbalances of the coagulation system (i.e., clotting factors) and of the fibrinolytic system (i.e., protein-dissolving activity by the body’s enzymes) cause a hypercoagulable state within the body and are thought to be the most important factors in bringing on an episode of acute DVT. A hypercoagulable state is a condition in which the blood is more readily coagulated or clotted than is normal. Thrombi usually begin in areas where coagulation is imbalanced and blood flow is stopped or decreased.

Each year, an estimated 116,000 to more than 250,000 acute DVTs occur in the United States. DVT is seen in the middle regions of the United States more commonly than on the west and east coasts. The mortality rate for hospitalized patients with DVT is 5%.