Endurance sports, such as cycling or distance running, put a significant amount of stress on an athlete's entire system, from the lungs to the bloodstream to the muscles. In order to maintain stamina, an athlete's muscles require steady supplies of oxygen contained within red blood cells. In theory, more red blood cells should give an athlete a natural edge during competition. This questionable training philosophy is behind an illicit practice called "blood doping," also known as "blood packing."
Blood doping involves harvesting an athlete's own blood before a competition or finding a matching blood donor. This blood is usually processed in order to create a concentration of red blood cells, then frozen until needed for transfusion back into the athlete shortly before the event.
The belief behind this practice is that the extra red blood cells will deliver more oxygen and other essential elements to the athlete's muscle tissues, which means more stamina and endurance. In a competitive sport such as cycling, the difference between winning and losing can often be a matter of better conditioning, not skill or strategy.
There are a number of drawbacks to the practice. If the athlete donates his or her own blood, a process called autologous transfusion, he or she may become anemic and unable to train at a competitive level. Blood from a matching donor, called a homologous transfusion, could contain a blood-borne illness or infection. An athlete who plans on receiving a homologous transfusion can continue to train hard until the day of the procedure, however.
Standard blood doping fell out of favor after a number of scandals exposed the practice to sport officials and sponsors. Although tests for it have never been proven especially accurate, many elite athletes and their trainers gave up the practice voluntarily. The practice of storing and processing red blood cells for later use became more and more difficult to hide from the general sporting community.
A newer form of blood doping has largely replaced the straight blood transfusion method, however. A growth hormone called erythropoietin (EPO), originally used to stimulate red blood cell growth in kidney patients, became the newest way for certain athletes to boost their own red blood cell counts before competition. Synthetic EPO can be injected into an athlete's body, where it stimulates bone marrow to produce more red blood cells. Only a very sophisticated drug test can differentiate between the legal natural EPO from the illegal synthetic.
A number of sporting events have been marred by the practice, or at least the suspicion, of blood doping. Random drug testing has discovered the illicit use of performance-enhancing drugs such as anabolic steroids, but accusations of blood doping can be notoriously difficult to prove. A blood test can show elevated levels of red blood cells, but many athletes have naturally higher levels because of legitimate training regimens and diets. More accurate tests are in development, but currently, the practice is most often proven through circumstantial evidence or eyewitness testimony from those who actually performed the transfusions or administered the synthetic EPO to an athlete already under suspicion.