width=260About a quarter of patients who have superficial blood clots on their legs may be at risk for more serious clots deep in their veins, Austrian researchers reported on Monday.

Varicose veins, oral contraceptives, injuries and cancer can cause blood clots near the surface of the legs.

Most cases are not serious, but this common condition could be a warning sign for deep vein thrombosis, in which a blood clot blocks off blood flow deep in the veins.

Deep vein thrombosis, or DVT, can kill if the clots move to the lungs, causing a pulmonary embolism.

Dr. Barbara Binder of the Medical University of Graz, Austria, and colleagues studied 46 patients with superficial vein clots from November 2006 and June 2007. All had a diagnostic imaging test and other lab tests.

The team found deep vein thrombosis in 24 percent of patients, even though most had no other symptoms.

Deep vein thrombosis most often occurred with superficial clots in the lower leg. In 73 percent of the patients, it occurred in the same leg as the superficial clot.

Our study ... demonstrates that the risk of a concomitant DVT should not be underestimated, Binder and colleagues wrote in the Archives of Dermatology.