Overview
Blood clots (thrombosis) form when blood thickens and clumps together inside blood vessels. A clot in a deep vein (deep vein thrombosis, DVT) most commonly forms in the leg. The most dangerous complication is when a clot breaks off and travels to the lungs (pulmonary embolism), which can be life-threatening.
How common is it?
DVT affects about 1 in 1,000 people each year in the UK. Pulmonary embolism accounts for around 25,000 hospital deaths annually.
Causes and risk factors
Clots form when blood flow slows, the blood itself is more prone to clotting, or blood vessel walls are damaged. This is summarised by Virchow's triad: stasis, hypercoagulability, and endothelial injury.
Common risk factors
- Prolonged immobility (long flights, bed rest, hospitalisation)
- Recent surgery, especially orthopaedic surgery
- Active cancer
- Pregnancy and the postpartum period
- Inherited clotting disorders (thrombophilia)
- Oral contraceptive pill or HRT
- Obesity
Symptoms
- Swelling, pain, tenderness, and redness in one leg (DVT)
- Warmth over the affected area
- Sudden shortness of breath (pulmonary embolism)
- Sharp chest pain that worsens with deep breaths
- Coughing up blood (pulmonary embolism)
- Rapid heartbeat and feeling faint
When to see a doctor
Go to A&E immediately if you have sudden breathlessness, chest pain, or cough up blood. See a doctor urgently for unexplained, painful leg swelling especially after surgery or travel.
Diagnosis
Blood D-dimer test screens for clots. Ultrasound scans confirm DVT. CT pulmonary angiography (CTPA) is the gold standard for diagnosing pulmonary embolism.
Treatments
Anticoagulant medication
Blood thinners (direct oral anticoagulants like rivaroxaban or apixaban, or warfarin) prevent clot growth and new clot formation. Usually prescribed for 3 to 6 months minimum.
Compression stockings
Below-knee compression stockings worn for at least 2 years after DVT reduce the risk of chronic leg swelling and pain (post-thrombotic syndrome).
Thrombolysis or surgical clot removal
Used for life-threatening massive pulmonary embolism when anticoagulation alone is insufficient. Clot-dissolving drugs (thrombolytics) are given to rapidly restore blood flow.
Self-care and lifestyle
- Move your legs regularly on long flights or car journeys, and stay well hydrated
- Elevate legs when resting after surgery
- Wear prescribed compression stockings as directed
- Report any leg swelling or breathing change promptly while on anticoagulants
Prevention
Preventive low-dose anticoagulants are given to most hospital patients at high risk. For long flights, walk regularly and consider compression socks. Discuss clot risk with your doctor before starting HRT or the pill.