Deep vein thrombosis (DVT) is the formation of a blood clot -- a thrombus -- within one of the deep veins of the body, most commonly in the veins of the calf, knee, thigh, or pelvis. While the clot itself is a serious condition, the greatest danger of DVT is its potential to break free and travel through the bloodstream to the lungs, causing a pulmonary embolism (PE) -- a medical emergency that can be fatal within minutes.
DVT and PE together are referred to as venous thromboembolism (VTE), one of the most common and preventable causes of hospital death globally. VTE affects approximately 1 in 1,000 people per year across the general population, with significantly higher rates in hospitalised patients, travellers on long-haul flights, post-operative patients, and those with cancer or inherited clotting disorders.
Deep Vein Thrombosis and Pulmonary Embolism: The Venous Thromboembolism Spectrum
DVT and PE are not two separate conditions -- they are two phases of the same disease process. Understanding their relationship is essential for recognising the full stakes of a DVT diagnosis.
- DVT begins when a clot forms in a deep vein. In many cases it causes localised symptoms: swelling, pain, and warmth in the affected limb. In approximately 30-50% of cases, DVT is completely asymptomatic and is only discovered when PE develops.
- PE occurs when the clot, or part of it, detaches and travels through the venous system to the right heart and then into the pulmonary arteries. Even a partial obstruction of pulmonary blood flow can cause severe breathlessness and haemodynamic compromise. A massive PE -- complete obstruction of the main pulmonary artery -- is rapidly fatal.
- The risk of PE is highest in proximal DVT (involving the popliteal, femoral, or iliac veins) compared with isolated distal DVT (confined to the calf veins), where the embolism risk is lower but not negligible.
The Mechanism: How DVT Forms (Virchow's Triad)
Blood clot formation in a vein is explained by Virchow's Triad -- three physiological conditions that, individually or in combination, predispose to thrombosis:
Venous Stasis (Slowed Blood Flow)
Leg muscles normally act as a pump, helping blood flow back to the heart. When a person is sitting or lying down for long periods, blood can pool in the deep veins. This increases the risk of blood clots (DVT), such as after surgery or during long-haul flights.
Endothelial Injury (Damage to Vein Walls)
The inner lining of veins normally helps prevent blood clots. When it is damaged by surgery, injury, inflammation, or IV cannulas, it can activate platelets and clotting factors, leading to blood clot formation.
Hypercoagulability (Increased Tendency to Clot)
Some people have a higher tendency to form blood clots due to inherited conditions or acquired factors. Examples include thrombophilia, cancer, pregnancy, antiphospholipid syndrome, and oestrogen-containing contraceptives. In these people, even minor triggers can lead to a DVT.
Symptoms of DVT
The classic presentation of DVT involves one leg becoming noticeably different from the other. Because DVT almost always affects one limb at a time, asymmetry is the most clinically useful sign.
Common Symptoms of Leg DVT
- Unilateral swelling: The most consistent sign; the affected leg is measurably larger in circumference than the other, particularly at the calf. Even mild swelling that develops over hours warrants evaluation.
- Pain or tenderness: Typically in the calf, behind the knee, or in the thigh. The pain may be described as aching, cramping, or heaviness. It may worsen with walking or standing but can also be present at rest.
- Warmth and redness: The skin overlying the thrombosed vein may feel warm to the touch and appear pink, red, or dusky purple depending on skin tone.
- Distended superficial veins: Collateral veins on the surface of the skin may become more visible as blood seeks alternative routes around the obstruction.
- Pitting oedema: Pressing a finger into the swollen lower leg leaves a temporary indentation.
What DVT can feel like without looking like much?
DVT is frequently less dramatic than the textbook description. Many patients describe only a mild aching in the calf that they initially attribute to a muscle strain. Some have only swelling without significant pain. The absence of severe symptoms does not exclude DVT -- and the absence of symptoms entirely does not exclude DVT either.
Symptoms of Pulmonary Embolism - A Medical Emergency
If you have any of these symptoms, seek medical attention immediately:
- Sudden onset of breathlessness: At rest, not explained by exertion
- Chest pain: Sharp, pleuritic (worse with breathing or coughing)
- Haemoptysis: Coughing up blood or blood-streaked sputum
- Syncope or near-syncope: Sudden faintness or loss of consciousness
- Rapid heart rate and low blood pressure: Signs of haemodynamic compromise
Risk Factors for DVT
Provoked DVT: identifiable triggering factors
- Recent major surgery: Particularly orthopaedic surgery (hip and knee replacement have among the highest DVT rates of any elective procedure), abdominal, pelvic, and neurosurgery
- Hospitalisation and prolonged immobility: Any acute medical illness requiring bed rest
- Long-haul travel: Flights or journeys exceeding 4-6 hours double the short-term DVT risk; the risk peaks 2-4 weeks after the flight
- Trauma: Fractures, particularly lower limb and pelvis fractures
- Pregnancy and the postpartum period: The risk of DVT is 5-10 times higher in pregnant women due to venous stasis, hypercoagulability, and vein compression
- Oestrogen-containing medications: Combined oral contraceptives and hormone replacement therapy increase DVT risk 3-6 fold
Other risk factors
- Age: Risk increases with each decade of life; DVT is uncommon under 40 without additional risk factors
- Obesity: Body mass index above 30 increases DVT risk by approximately 2-3 fold, through increased intraabdominal pressure and adipose tissue-derived prothrombotic factors
- Previous DVT or PE: The strongest single predictor of future VTE
- Varicose veins: Associated with superficial thrombophlebitis and a modestly increased DVT risk
- Heart failure and chronic lung disease
- Dehydration: A contributing factor in travel-related DVT, compounded by alcohol and caffeine
How DVT is Diagnosed
- Ultrasound: The main test used to diagnose DVT. It uses ultrasound to examine the deep veins of the leg. A normal vein compresses when pressed, while a vein containing a clot does not. It is safe, painless, non-invasive, and does not use radiation.
- D-Dimer Blood Test: This blood test detects a substance released when blood clots break down. A negative result can help rule out DVT in patients with a low or moderate risk of having a clot. A positive result does not confirm DVT, as D-dimer can also be raised by infection, inflammation, cancer, or recent surgery. Further imaging may be needed.
- CT Venography: A CT scan using contrast dye to look at the veins and detect blood clots. It is mainly used when DVT is suspected in areas that are difficult to assess with ultrasound, such as the pelvis, abdomen, or upper limbs. It may also be used when an ultrasound is unclear or when a CT scan for pulmonary embolism (PE) is being performed at the same time.
- CT Pulmonary Angiography: The main scan used when pulmonary embolism (PE) is suspected. It uses contrast dye to directly detect blood clots in the lung arteries. It can sometimes be combined with CT venography to check for both DVT and PE.
DVT Prevention
For hospitalised and post-surgical patients
- Medication: Blood-thinning medicines such as LMWH (enoxaparin) or fondaparinux can help prevent DVT in patients at moderate to high risk, especially after major surgery.
- Mechanical methods: Compression stockings and intermittent pneumatic compression (IPC) devices help keep blood flowing in the legs, especially when blood-thinning medication cannot be used.
- Early mobilisation: Walking from day one after surgery, where clinically safe, is the most effective mechanical prevention strategy.
Lifestyle measures
- Maintain a healthy weight: Obesity is a significant, modifiable DVT risk factor
- Exercise regularly: Active lower limb muscles act as a venous pump, maintaining blood flow
- Avoid prolonged sitting without breaks: Set a reminder to move every hour at sedentary workloads
- Stay hydrated: Dehydration concentrates the blood and increases clotting tendency
- Do not smoke: Smoking damages vascular endothelium and promotes hypercoagulability
- Review medications with your doctor: If on oestrogen-containing contraceptives and planning major surgery or a long trip, discuss temporary alternatives or bridging strategies
