Editor,—It is somewhat self contradictory to suggest that “a transferrin saturation of <30% may help the diagnosis” if there is still doubt about validation of iron deficiency after receipt of the serum ferritin result, the authors having previously acknowledged that the latter is “the most powerful test for iron deficiency” (Gut2000;46(suppl IV):iv1–5). Statistical considerations which dictate that serum ferritin will always outrank transferrin saturation in predictive power have their basis in the comparison between the receiver operating characteristic (ROC) curves for serum ferritin versus transferrin saturation, yielding values of 0.91 versus 0.71 (p<0.001) for the area under the curve.1Statistical considerations also dictate acknowledgement of mean corpuscular haemoglobin (MCH) as a predictive entity …