When it affects a larger, deeper vein and a blood clot forms, the doctor may diagnose the patient with deep vein thrombosis (DVT). There are no visible symptoms because muscle covers the veins. Only half of the people with DVT show any symptoms at all until serious complications develop. Pain may occur in the calf or thigh and may be more noticeable in the foot while walking or flexing. DVT is a more serious issue than superficial phlebitis because a blood clot could break loose and travel through the bloodstream to the lungs.

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