A study conducted by a team of researchers led by Dr. Barbara Binder of the Medical University of Graz, Austria indicated that roughly a quarter of patients who have superficial blood clots on their legs may be at risk of developing more serious clots deep in their veins. Deep vein thrombosis (DVT), in which a blood clot blocks off blood flow deep in the veins, can prove fatal if the clot shifts to the lungs, causing pulmonary embolism.