Swelling in one leg: causes, red flags and when to get it checked

Fluid builds up in one leg when blood or lymph is blocked, when the skin and tissues are inflamed, or when an injury has caused bleeding. Because swelling that affects one leg is more likely to have a local cause, it is investigated differently to swelling in both legs.

• Injury, muscle tear or ruptured Baker cyst behind the knee, with bruising and pain after exertion.

• Cellulitis: a red, hot, tender area, often with fever, sometimes after a cut or athlete foot.

• Deep vein thrombosis: swelling, calf pain, warmth and sometimes dilated surface veins, risk factors including recent surgery, long travel, pregnancy, hormones, cancer and previous clots.

• Chronic venous insufficiency and varicose veins, with heaviness, aching and skin changes.

• Lymphoedema, which may follow surgery, radiotherapy, infection or cancer.

• Arthritis or gout in the knee or ankle, with joint swelling.

• Pelvic mass or lymph node enlargement compressing veins.

• Compartment syndrome after injury, and rarely tumours of soft tissue.

Sudden swelling with severe pain, tightness and shiny skin, a pale, cold or numb leg, breathlessness, chest pain, coughing blood, fever and rigors, redness spreading quickly, or a history of cancer, previous clots or recent surgery suggest urgent causes.

• Call 999: leg swelling with breathlessness, chest pain, coughing blood or collapse; or a cold, pale, numb leg.

• Same-day assessment (urgent care, A&E or GP): any new one-sided leg swelling, calf pain or redness, particularly after travel or surgery.

• Same-day GP: a hot, red, rapidly spreading area of skin with fever.

• GP within days: long-standing one-sided swelling with no pain.

Doctors ask when swelling began, pain, injury, recent travel, surgery, immobility, hormones, pregnancy, cancer history, previous clots, fever and skin breaks. They measure calf circumference, assess warmth, tenderness, redness, veins, pulses and the other leg, and score the probability of a clot.

• D-dimer blood test: a normal result with a low clinical score helps exclude a clot.

• Leg vein Doppler ultrasound: the key test to see a clot in the deep veins.

• Blood tests for infection, kidney function, liver function and albumin.

• CT pulmonary angiogram: if breathlessness or chest pain suggests a clot in the lungs.

• Ultrasound or MRI of the knee and calf for Baker cyst or muscle tear.

Do not massage the calf if a clot is suspected. Rest with the leg raised while arranging assessment, and walk if it is comfortable once a clot is excluded. Stay hydrated on long journeys and move regularly. Treatment for a confirmed clot is with anticoagulants prescribed by a doctor, and for cellulitis with antibiotics.

On the NHS, a same-day DVT pathway through urgent care, A&E or GP allows a scan within a day. Private hospitals offer rapid Doppler ultrasound, but emergency symptoms need 999.