What it means
Thrombosis is the formation of a blood clot — a thrombus — inside a blood vessel, obstructing flow through it.
Clotting is a normal and necessary response to injury. Thrombosis is that mechanism operating inappropriately, within an intact vessel.
The classical framework describing why it occurs is Virchow's triad: reduced blood flow, injury to the vessel wall, and increased tendency of the blood to clot. Immobility, surgery, pregnancy, cancer, oestrogen-containing medication and inherited clotting disorders all act through one or more of these.
Arterial thrombosis typically causes ischaemia and infarction; venous thrombosis typically causes swelling and pain, and carries the risk of embolism.
In practice
Encountered as deep vein thrombosis, cerebral venous thrombosis and coronary thrombosis, among others.
Investigation commonly involves a D-dimer blood test and ultrasound or CT imaging. Treatment is with anticoagulant medication, sometimes described as blood-thinning although the blood is not literally thinned.
Why it matters
A swollen, painful, warm calf raises the possibility of deep vein thrombosis and requires same-day assessment. Chest pain with breathlessness may indicate that a clot has travelled to the lungs.
Prevention is a routine part of hospital care, and includes early mobilisation, compression stockings and preventive anticoagulation after surgery.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026