Deep Vein Thrombosis

Vascular DVT

A blood clot in a deep vein — usually the leg — that blocks the return of blood and can break off to the lungs.

Educational summary only — not medical advice, and no substitute for assessment by a clinician. Diagrams are simplified illustrations.

A clot in a deep leg vein blocks blood's return, so fluid backs up and the leg below swells, warms and aches.

Overview

Deep vein thrombosis is a blood clot that forms in one of the body's deep veins, most often in the calf or thigh. Veins carry blood back toward the heart, so a clot here partly or fully blocks that return flow. DVT matters for two reasons: the local swelling it causes, and the risk that a piece breaks loose and travels to the lungs.

How the system normally works

Deep veins run alongside the bones, wrapped by muscles. When you walk, the calf muscles squeeze these veins and pump blood upward; one-way valves stop it falling back. This 'muscle pump' keeps blood moving against gravity.

What goes wrong

Clots tend to form when three conditions combine (Virchow's triad): blood that sits still (long flights, bed rest, surgery), a damaged vein lining, and blood that clots too readily. A clot grows inside the vein and blocks flow. Blood and fluid then pool below it, swelling the limb. Because the clot is only loosely attached at first, a fragment can detach and be carried to the lungs.

Diagram of the three drivers of clot formation in a vein: slow blood flow, damaged vein lining, and increased clotting tendency, with a clot forming behind a valve. — Clots form when three things line up: sluggish blood flow, an injured vein lining, and 'stickier' blood — often pooling behind a vein valve.

Symptoms and why they happen

Swelling of one leg (often the calf) — blood and fluid backing up behind the clot.
Aching or cramping pain, worse on standing or walking.
Warmth and red or darkened skin over the area — local inflammation.
Tightness or heaviness in the leg.
Many DVTs cause only subtle symptoms, so a high index of suspicion matters.

Causes and risk factors

Immobility: long travel, hospital stays, surgery (especially hip/knee)
Recent injury or fracture of the leg
Cancer and some cancer treatments
Pregnancy and the weeks after birth; estrogen-containing contraception
Obesity, smoking, increasing age
Inherited clotting tendencies

How it's diagnosed

A scoring system (the Wells score) estimates probability, combined with a D-dimer blood test that, if normal, helps rule out clot. The confirming test is a Doppler ultrasound of the leg, which shows whether the vein compresses normally or is blocked by clot.

Diagram showing a clot breaking off a leg vein and travelling up through the heart to the lung arteries as a pulmonary embolism. — The major danger of a DVT: a fragment can break off, travel up through the heart, and lodge in the lungs as a pulmonary embolism.

Treatment and management

The mainstay is anticoagulation ('blood thinners') — modern tablets (DOACs) or injections — which don't dissolve the clot but stop it growing and prevent new ones while the body slowly reabsorbs it. Treatment usually lasts at least three months. Large or limb-threatening clots may need clot-busting drugs or a procedure. Compression stockings and staying mobile help symptoms.

Possible complications

The acute danger is pulmonary embolism. Longer term, a damaged vein can cause 'post-thrombotic syndrome' — chronic leg swelling, aching, and skin changes.

Prevention and outlook

On long journeys, move your legs, walk regularly, and stay hydrated. Around surgery, hospitals use blood thinners, compression devices, and early walking. With prompt treatment, most people do well, though some need longer or lifelong anticoagulation depending on the cause.

When to seek emergency care

Sudden shortness of breath, chest pain, or coughing up blood — the clot may have reached the lungs.
A leg that becomes severely swollen, painful, pale or blue — a rare limb-threatening clot.