Serum ferritin is one of the most important and widely used markers of iron metabolism, and is a recommended standard index of iron dynamics in guidelines for renal anemia in many countries. However, serum ferritin measurement has several disadvantages. For example, it is prone to being influenced by underlying disease, and there is considerable systematic bias among different methods of measurement.Systematic bias affects not only the measurement results, but also the therapeutic strategy to be used in accordance with the relevant guidelines, and consequently the cost of medical treatment. Key Messages: Manufacturers should reduce variability in their serum ferritin assay kits. Researchers should take into account propagation of error in the analysis of measurement values, and clinicians should evaluate laboratory data with care.