Blood clots (or phlebitis) can present as red tender lines on the legs or they can develop silently and grow extensively in the deep veins of the leg before being discovered. They are suspected by the physician when there is swelling or pain in just one leg that is not explainable by any other cause such as an injury or other event.
The diagnosis requires an ultrasound examination for positive identification. This should be done for any extremity in which unexplainable symptoms of swelling or pain occurs. Phlebitis occurs in superficial veins just under the skin or deep veins in the muscles of the legs. Superficial phlebitis is easily seen as an area of inflammation that is tender and red and swollen and feels usually like an oblong lump near the skin. It often occurs in legs that have varicose veins. This kind of phlebitis is easier to diagnose and less serious than the phlebitis that develops in the deep veins where it has a high tendency to extend up and down the leg and give rise to pieces that break off and travel to the lungs.