- Laboratory analyses: Complete blood counts will be determined in an automated cell counter. Percentage and absolute reticulocyte count will be obtained locally by an automated cytofluorometric method. Serum erythropoietin (Epo) levels will be measured centrally in Liège on stored samples by a commercially available radioimmunoassay (Incstar Corp., Stillwater, MN, USA). Based on regression equations obtained in appropriate reference subjects between Hct on the one hand and log (Epo) on the other, predicted log (Epo) values are derived for each Hct and O/P ratios of observed/predicted Epo values will be calculated. Serum soluble transferrin receptor (sTfR), a quantitative measure of total erythropoietic activity, will be measured centrally in Liège on stored samples by a commercially available ELISA (R&D, Minneapolis, MN, USA). Normal values range from 3,000 to 7,000 µg/L. Serum iron, transferrin saturation and ferritin, as well as standard biochemical values, will be measured locally by standard methods.