Endurance performance depends on the oxygen transport system of the body. One method to increase this capacity is to train at high altitudes when preparing for competitions at sea level. The lower pressure of oxygen at altitude stimulates the production of red blood cells in the body.
Another method are EPO and other blood doping methods which enhance the oxygen transport capacity of the blood, too. Athletes in endurance sports continue to use such practises to enhance their performance.
The abuse of recombinant EPO (rhEPO), a synthetic version of the naturally occuring erythropoietin, or blood transfusions increases the number of the red cells in the circulation.
Excessive red blood cells increase the risk of blood clots due to higher viscosity of the blood, even more so in combination with the training induced slow heart rate and low blood pressure of endurance athletes. Further, the blood, already thicker with the extra red blood cells, might become even thicker during heavy exercise at high temperatures as fluid is lost from the body. Therefore, these methods carry the risk of fatalities like stroke or heart attack.
In the 1970s, when blood doping was first studied, blood transfusions had been common practice, however, this method virtually disappeared when rhEPO became available on the market in 1987. Unfortunately, the doping test for EPO introduced in 2000 has had a most unwanted effect: a return to the "ancient" method of blood transfusions. But the detection methods used by anti-doping laboratories permanently improve, as does the targeting of potential abusers.