Thrombosis is the formation of an abnormal mass known as a blood clot or ‘thrombus’ inside a blood vessel that significantly reduces, or stops the flow of blood1.

There are three broad categories of factors that disturb the normal clotting process and contribute to thrombosis2. These are:

  • Hypercoagulability—a rare tendency for blood to clot more easily than usual, caused by changes to the contents of the blood.
  • Changes in blood flow—alterations to normal blood flow, such as venous stasis in which blood flows slowly in the legs, for example after an operation when patients are immobile.
  • Endothelial dysfunction—injury to the lining of blood vessels, for example caused by surgery, or abnormal endothelial changes, caused by some metabolic illnesses2.

Doctors often refer to these categories as ‘Virchow’s triad’ and still use these principles today when diagnosing and treating patients with thrombosis2.

What is Venous Thromboembolism (VTE)?

VTE is a term that is commonly used to describe an evolving, multifactorial thrombosis disease spectrum2.

What is Deep Vein Thrombosis (DVT)?

The formation of blood clots in the deep veins, mainly of the legs. This is the most common form of VTE1.

Legs have two sets of veins—superficial veins and the deep veins that drain blood from the leg muscles. A deep vein thrombosis (DVT) is a blood clot that forms in the veins deep in the leg, usually in the calf or thigh, although DVT can occur in other veins including the pelvis4. DVT can block the flow of blood partially or completely and cause many or no symptoms.

Most DVTs form in the calf and about one fifth will extend into the thigh5. Doctors refer to DVT as distal or proximal; distal DVT refers to DVT of the calf, proximal DVT refers to DVT above the knee and above a major vein known as the popliteal vein. Proximal DVT has been associated with more serious complications than distal DVT because this DVT further up the leg is more likely to embolise and the emboli are potentially larger4.

Symptoms of DVT

In many cases of DVT, the clots are small and do not cause any symptoms. The body is able to gradually break down the clot and there are no long-term effects. However, if the clot is large and blood flow is partially or totally blocked in the vein, the following symptoms can occur:

  • Swelling—this is usually different from the mild ankle swelling that many people get during long haul flights, for example; if DVT occurs in the thigh, the whole leg may swell
  • Pain that is noticeable or worse when standing or walking
  • Discolouration of the leg, which can appear a blue or red/purple colour
  • Increased warmth in the lower leg 5,6
Complications of DVT

DVT might not cause any further problems, but potential complications associated with DVT can be serious and unpleasant and include:

  • Pulmonary embolism (PE) (see the next section)—when part of the blood clot breaks off and blocks blood flow to a lung 3, 6
  • Recurrence of venous thromboembolism (DVT or PE) 6
  • A condition called post-thrombotic syndrome that develops in up to one half of patients after symptomatic DVT and is the most common complication of DVT
    • Typical features include chronic pain, swelling, a feeling of heaviness, skin changes in the affected leg and fluid retention.
    • The symptoms are usually worse with activity; for example, standing and walking
    • Severe post-thrombotic syndrome may induce unpleasant leg ulceration.6

What is Pulmonary Embolism?

A blood clot that blocks the blood vessels supplying the lungs. The clot most often comes from the legs and travels through the heart to the lungs 3.

Pulmonary is a term used when referring to something affecting, occurring in, or related to the lungs. When a blood clot (thrombus) detaches from the wall of the vein in which it has formed, it is carried in the blood through the vasculature. Subsequently, the thrombus can pass through the right ventricle of the heart, which forces it into the pulmonary circulation (circulation to the lungs), where it can lodge. This is known as a pulmonary embolism (PE) 7.

Symptoms of PE

Although common symptoms of a PE include breathlessness and bloody mucus, not all patients present with these symptoms3. Doctors employ an assessment score that has been shown to accurately assess the chance of having a PE on the basis of a number of measurements.

Complications of PE

PE is a very serious condition, which can have severe consequences including damage to the right ventricle of the heart (ventricular failure), hypertension, and death.7 Therefore, guidelines for doctors recommend that patients waiting for a test for PE are given treatment even before the diagnosis is confirmed.8


  1. Kesieme E, Kesieme C, Jebbin N et al (2011). Deep vein Thombosis: a clinical Review. J Blood Med, 2, 59–69. doi: 10.2147/JBM.S19009
  2. Cross M, Boettner F (2009). Pathophysiology of Venous Thromboembolic Disease. Seminars In Arthroplasty, 210 – 216.
  3. Jill M. Merrigan, BA; Gregory Piazza, MD, MS; Cassio Lynm, MA; et al (2013). Pulmonary Embolism. JAMA, 309 - 504. doi:10.1001/jama.2012.145097
  4. Osman A, Ju W, Sun D et al (2018). Deep Venous Thrombosis: a literature review. Int J Clin Exp Med, 11 (3), 1551 – 1561.
  5. Stubbs M J, Mouyus M, Thomas M (2018) . Deep Vein Thrombosis. BMJ, 360. doi:
  6. Ginsberg J (2012). Goldman’s Cecil Medicine (24th edition): Peripheral Venous Disease.
  7. Konstantinides V, Meyer G, Becattinin C et al. 2019 ESC guidelines for diagnosis and management of acute pulmonary embolism developed in collaboration with the European respiratory society (ERS). European Heart Journal, 2020, 41, 543 – 603. Doi: 10.1093/eurheartj/ehz405
  8. NICE guidelines (2020). Venous Thromboembolic diseases: diagnosis, management and thrombophilia testing.

MAT-GB-2000453 (v5.0) Date of preparation: March 2022