The race to keep up with blood doping sports cheats
Scientists have developed tests to detect nearly every type of doping in sport, but they have not yet perfected a way to prevent all forms of the banned practice of blood doping.
It involves athletes storing and transfusing their own blood - or the blood of others - before exercise or competition, to increase the number of red blood cells in the body.
By manipulating their oxygen-carrying capacity, cheats can increase their stamina and speed.
It is possible to detect some - but not all - types of blood doping.
Video produced by Thomas Hannen.