With the practice of blood doping among athletes becoming more popular, more research is necessary to devise new methods of detection

Dr Eduardo Rocha, researcher

This, in turn, allows the athlete to carry more oxygen from the lungs to the muscles, boosting performance, particularly in endurance events.

However, because of the increased proportion of red cells, the blood becomes more dense and viscous.

This, it is thought, makes it more likely that blood clots will form and block blood vessels - which may have been what happened in this case.

Spanish doctor Eduardo Rocha, from the Clinica Universitaria de Navarra, said: "With the practice of blood doping among athletes becoming more popular, more research is necessary to devise new methods of detection, as well as more thorough evaluation of competitive athletes seeking treatment for potential doping complications.

"Educating athletes about the potential health hazards of performance enhancing drugs would also be beneficial."

Scandal source

EPO use created a major scandal in the world of competitive cycling after one of the Tour de France's top teams, Festina, was expelled from the 1998 race following drugs finds.

The latest positive tests in Salt Lake City involved cross-country skiers, and a new variant of EPO called darbepoitin, which, it is claimed, may be many times more potent.

John Brewer, from the National Sports Science Centre at Lilleshall in Shropshire, said that the risks involved with EPO use had been highlighted before.

He said: "With EPO the effect is very similar to training at altitude, but there is the tendency for the blood to become more viscous."