Venous Thromboembolism and Its Risk Factors

Venous thromboembolism (VTE) is a condition in which blood clots form in the veins. Up to 900,000 VTE events occur each year in the United States. VTE is most common in older adults, but it can occur at any age.

What is VTE and how does it happen?

VTE develops when blood flow slows down, a vein is damaged or the blood becomes more likely to clot. These changes can happen for many reasons and often occur together.

VTE includes two related conditions:

• Deep vein thrombosis (DVT): A blood clot that most often forms in the deep veins of the lower extremities or pelvis but sometimes in the arm or other veins. A DVT can develop when blood flow slows, the vein wall is injured or the blood becomes more likely to clot. Major risk factors include surgery, trauma, cancer, long periods of immobility and serious medical illnesses.
• Pulmonary embolism (PE):  A condition that occurs when part of a blood clot that forms in a deep vein breaks loose from a vein wall, travels to the lungs and blocks blood flow in the pulmonary arteries of the lungs. Blood clots starting in the thigh are more likely to travel to the lungs than blood clots in the lower leg or other parts of the body. Learn more about a pulmonary embolism.

VTE risk factors

Major risk factors for VTE include cancer, surgery or major trauma. A hospital stay can increase VTE risk, especially when it involves reduced mobility, serious illness or major procedures. Anyone with one of these risk factors should ask their health care professional whether preventive treatment is appropriate.

Some VTE risk factors are related to specific situations:

• Major surgery or hospitalizations — Blood clots after surgery can form in the deep veins of the legs or pelvis.
• Major trauma, such as lower-extremity paralysis due to spinal cord injury or fracture of the pelvis, hip or long bones
• Cancer — Tumor cells can release substances that activate the clotting system and increase the risk of blood clots.
• Prolonged immobility, such as during long-distance travel (more than four or five hours), can slow blood flow in the legs. The risk of VTE is highest in the first one to two weeks after travel. It is greater if other risk factors are present.

Additional VTE risk factors

On their own, these factors usually don’t require preventive treatment. When they occur together or with other risk factors, they may inform decisions about prevention.

• Prior VTE — People with a previous episode of VTE have a high chance of another.
• Age — Risk increases steadily with age, especially after 40.
• Obesity — People living with obesity have an increased risk of VTE. The higher the body mass index, or BMI, the higher the risk.
• Other chronic medical conditions, such as heart failure, kidney disease, lung disease or inflammatory bowel disease
• Oral contraceptives or estrogen treatment for menopause symptoms may increase the blood’s tendency to clot by affecting clotting proteins.
• Family history of VTE, especially if in a first-degree relative (parent, sibling, child)
• Physical inactivity
• Acquired or inherited coagulation disorders
• Smoking
• Some nutritional deficiencies, such as folate or vitamin B12, can raise homocysteine levels. This has been associated with increased clotting risk, though the link is less well established than for other risk factors.

VTE and pregnancy

Risk for a VTE is higher in pregnancy and highest in the first six weeks after delivery. The risk is greater when combined with other factors, including:

• History of blood clots
• A genetic tendency for VTE or a family history of VTE (especially in a first-degree relative, such as a parent, sibling or child)
• Obesity
• Limited mobility, including bed rest and long-distance travel
• Multiple births
• Older maternal age
• Other medical illness during pregnancy, including cancer, serious infection or pre-eclampsia