Deep vein thrombosis (DVT) is the formation of a thrombus (blood clot) within a deep vein, most commonly in the lower extremity. Compression ultrasound is the primary diagnostic test: a normal vein collapses completely when the transducer is pressed against the skin, while a thrombosed vein is non-compressible. Color Doppler demonstrates absent or reduced flow in thrombosed segments. DVT is a significant source of morbidity because thrombus can embolize to the pulmonary circulation, causing pulmonary embolism.