Red, swollen leg

A leg that becomes swollen, red, warm and painful — especially when it affects one leg — is a symptom that always deserves attention, because one of the important causes is a deep vein thrombosis (DVT): a blood clot in a deep vein of the leg. A DVT typically causes swelling, aching or cramping pain (often in the calf), warmth and sometimes redness in one leg, and it matters because part of the clot can break off and travel to the lungs (a pulmonary embolism), which is dangerous. Another common cause of a red, warm, tender leg is cellulitis — a bacterial skin infection — which often comes with fever and a spreading area of redness. Other causes include a ruptured cyst behind the knee, an injury or muscle tear, and, when both legs swell rather than one, more general causes such as heart, kidney or liver problems, or venous insufficiency. The distinction that matters most is whether it is one leg (raising the concern of a DVT or infection) or both (pointing to a systemic cause), and whether there is pain, warmth and redness. Because a DVT can be serious and is treatable, a newly swollen, painful leg should be assessed promptly.

• Deep vein thrombosis (DVT) — a clot in a deep leg vein; typically one leg, with swelling, pain and warmth.
• Cellulitis — a bacterial skin infection; redness, warmth, tenderness, often with fever.
• A ruptured cyst behind the knee (Baker's cyst) — can mimic a DVT.
• Injury or a muscle tear.
• Venous insufficiency or chronic swelling — often both legs, longer-standing.
• Heart, kidney or liver problems — usually swelling of both legs.

A newly swollen, painful leg is assessed clinically first, and if a deep vein thrombosis is possible, this is investigated promptly. A D-dimer blood test reflects active clotting and is used, together with a clinical score, to help decide who needs an ultrasound scan of the leg veins — the test that confirms or excludes a DVT. If infection (cellulitis) is likely, inflammatory markers (CRP) and the white cell count support the picture, and treatment is with antibiotics. If a DVT is confirmed, especially if unprovoked or recurrent, a thrombophilia work-up may follow later. Mediora.AI can surface a raised D-dimer or inflammatory markers that fit the picture, but a newly red, swollen, painful leg needs prompt clinical assessment — and often an ultrasound — by a doctor, because a DVT is both potentially serious and treatable.