Blood clotting (thrombosis)
Thrombosis is a general term for blood clotting.  Deep Vein Thrombosis, (DVT), is the term given to clots that occur in the deep venous system. Most of these clots occur in the legs. About a half-million people are treated each year in the United States for DVT. When blood clots form in veins, they block blood flow, causing swelling, pain and warmth. Occasionally, a part of the clot may break off and travel through the blood stream into the lungs. This is called a pulmonary embolus (PE) and can be life threatening. Symptoms of PE can include shortness of breath with or without activity, increased and/or irregular heart rate, chest pain, sudden loss of consciousness or coughing up blood.

Causes of blood clotting
Your body forms blood clots when a blood vessel is injured. Our bodies naturally keep the clotting process in check. However, if one of the mechanisms that help stop clotting is not working correctly, it may result in abnormal clotting. Inherited (genetic) disorders contribute to about one-half of all clotting episodes that occur in a vein. They are suspected if clotting occurs in someone who is relatively young, has occurred in many sites, or occurs in unusual locations in the body. There are also non-inherited risk factors for blood clotting.

Inherited risk factors

• Factor V Leiden-Common, affecting 5 percent of general population, 20-40 percent of people with vein clots
• Prothrombin Gene Mutation-Common, affecting 2-3 percent of the general population
• Protein C Deficiency-Rare
• Protein S Deficiency-Rare
• Antithrombin III Deficiency-Rare
• Dysfibrinogenemia-Rare
• Homozygous homocystinuria-Very rare
• History of clotting, along with a strong family history of clotting, about 50 percent of cases

Non-inherited risk factors

• Pregnancy
• Oral contraceptive use
• Cancer
• Trauma
• Major surgery
• Prolonged bed rest or inactivity
• Presence of a lupus anticoagulant
• Hormone-replacement therapy
• Previous thrombosis
• Extreme obesity
• Advancing age (Vein clotting increases from in 1 per 100,00 during childhood to 1 per 100 in people 75 years old and over)
• Possibly lengthy air travel

Symptoms
About half of those with DVT have no symptoms until a clot blocks a major vein. When a DVT blocks blood flow, symptoms may include:

• General swelling in the calf, ankle, foot or thigh
• Increased warmth of the leg
• Redness
• Pain in the leg 
• Leg cramps
• Bluish discoloration of the skin on the leg or toes

Diagnosis
Your health care provider will ask about your medical history and your symptoms. Your provider will examine you, with special attention to any abnormal areas, such as a swollen leg. Sometimes the pain can be felt deep in the calf or thigh. Your legs may be measured to compare sizes on the right and left. Tests are needed to confirm the diagnosis. Blood clots are sometimes discovered by doing a CT (computed tomography) scan of the pelvis. The most common tests are impedance plethysmography (IPG), duplex ultrasound and contrast venography.

• IPG measures vein function in the arm or leg. Your health care provider or a technologist will place a pressure cuff on your arm or leg and measure how fast the veins empty.
• Duplex ultrasound bounces sound waves off the deep veins in an arm or leg to create pictures which help locate any blockages, and measure how fast the blood flows through the veins.
• Contrast venography is used when other tests don't give a definite answer. A special dye is injected into a vein while x-ray pictures are taken. It usually shows any blockages in your veins.

Treatment
A person diagnosed with a clot is put on blood thinners as soon as possible to prevent the clot from getting larger and reduce the chance that it will break loose and travel to the lung. In some cases, medications are used to dissolve the clot directly. Blood thinners most commonly used are heparin and warfarin. Generally, heparin is used first, given by infusion or under the skin.  Warfarin is then started, and once it is up to a desired level in the blood, the heparin is stopped. Blood thinning with warfarin may be needed for several months or indefinitely, based on the circumstances of the clot and if any inherited tendencies are identified.