The swollen leg: DVT versus cellulitis, a case guide

The case: one hot, swollen calf

Our illustrative patient is a man in his sixties who returns from a long-haul flight and, two days later, notices his left calf is swollen, warm and aching, especially when he walks. There is no obvious cut or spreading redness, and he feels well in himself. A single swollen calf after prolonged immobility is a classic setting for DVT. In another version of the story, the same leg might instead be bright red, hot, tender and spreading, with a broken area of skin and the patient feeling feverish and unwell — a picture that points towards cellulitis. The clues lie in the pattern, the skin and how ill the person feels.

What DVT is and why it matters

A deep vein thrombosis is a blood clot that forms in one of the deep veins, usually in the calf or thigh. It typically causes swelling, aching, warmth and sometimes a bluish or reddish tinge in one leg. It matters because part of the clot can break off and travel to the lungs, causing a pulmonary embolism — sudden breathlessness, chest pain or coughing up blood, which is a medical emergency. Risk rises with long periods of immobility, recent surgery, cancer, pregnancy, some hormone treatments and inherited clotting tendencies. Prompt diagnosis and blood-thinning treatment greatly reduce the danger, which is why a suspected clot is always assessed quickly rather than left to settle on its own.

What cellulitis is and why it matters

Cellulitis is a bacterial infection of the skin and the tissues just beneath it. It usually causes a spreading area of redness that is hot, swollen and tender, often with a raised temperature and feeling generally unwell. Bacteria commonly enter through a break in the skin — a cut, insect bite, athlete's foot between the toes, or a leg ulcer. It nearly always affects one leg. Untreated, the infection can spread and make people seriously ill, so it needs antibiotics, usually taken by mouth, with more urgent hospital care if it is severe, spreading rapidly or affecting the face. Marking the edge of the redness helps clinicians judge whether it is improving.

How clinicians tell them apart

Both conditions typically affect one leg, which is why they are confused, but there are useful differences. Cellulitis tends to have clearly spreading redness with a defined edge, obvious skin warmth, a possible entry wound and a fever, and the person often feels unwell. DVT more often causes deep aching and swelling with less dramatic surface redness, and is strongly linked to recent immobility or clotting risks. Because the history matters so much, clinicians use structured risk scoring for DVT, alongside a blood test (D-dimer) and an ultrasound scan of the leg veins. Rarely, the two can occur together, so careful assessment is essential.

Getting the right help quickly

Because these conditions need different treatments, a swollen, painful leg should be assessed promptly rather than watched at home. Contact your GP or NHS 111 the same day if one leg becomes swollen, warm or painful without clear cause, or if an area of skin turns red, hot and spreading. Go to A&E or call 999 if leg symptoms come with sudden breathlessness, chest pain or coughing up blood, which may mean a clot has reached the lungs. Early treatment — blood thinners for a clot or antibiotics for infection — is safe and effective, and quick action prevents serious complications.