DEEP vein thrombosis (DVT) is a blood clot formed in a deep vein, usually in the calf.

Symptoms include swelling, pain and reddening of leg, but not all patients experience symptoms.

The most serious complication is a pulmonary embolism, where part of a clot breaks off and travels to the lungs, where it can then block key arteries. This can be fatal.

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Many DVT clots are relatively small and the body can break them down with no long-term effects. They can occur for no apparent reason but are associated with periods of inactivity - such as sitting on long-haul flights or staying in bed to recover from surgery.