Disturbances of iron metabolism, particularly iron deficiency and iron redistribution are among the most commonly overlooked or misinterpreted diseases. This is due to the fact that the determination of transport iron in serum or plasma, which used to be the conventional diagnostic test, does not allow a representative estimate of the body’s total iron reserves. In the past a proper estimate was only possible by performing costly and invasive determination of storage iron in the bone marrow. However, sensitive, well-standardized immunochemical methods for performing a precise determination of the iron storage protein ferritin in plasma are now available. Since the secretion of this protein correctly reflects the iron stores in the majority of cases, these methods permit fast and reliable diagnoses, particularly of iron deficiency status.