The present study is meant to define the laboratory parameter (%-Hypo/CHr or RET-He) which is suited best to diagnose functional iron deficiency. The study design asks for the parameter with which physicians will be able to diagnose their patients so to improve the management of their anemia. A diagnostic parameter is searched for which improves the patients' treatment the most, as measured by blood hemoglobin levels (primary end point 1), at the lowest possible costs (primary end point 2).