In XC skiing they noticed that competitors' haematocrits - the number of red cells in the blood - were unphysiologically high - so they set an upper limit above which they could not compete - all of a sudden the haematocrits were just below this level. In cycling they also keep an eye on the haematocrit, and according to Wiki: "The Union Cycliste Internationale (UCI), for example, imposes a 15-day suspension from racing on any male athlete found to have an HCT above 50% and hemoglobin concentration above 17 grams per deciliter (g/dL)." As has been suggested, this would encourage the use of small doses of EPO, just enough to nudge the haematocrit towards, but not above 50. Why else would the teams need a centrifuge? Exercise by itself reduces the haematocrit.