BACKGROUND: The purpose of this study was to estimate the value of the transferrin receptor in serum (sTfR) for detecting iron deficiency and compare it with the value of some other blood tests for that purpose. MATERIAL AND METHODS: All patients undergoing bone marrow aspiration in the FSA Hospital, Akureyri, Iceland, in the period 1999 to 2003 were eligible to participate in this prospective study. Included participants were 89. The sensitivity, specificity, efficiency, and Youden index of ferritin, MCV, CHr, sTfR, sTfR-Ferritin-index, the iron saturation of transferrin, and the Thomas-Plot method were calculated. The complete absence of stainable iron in bone marrow was used as the definitive marker of iron depletion. RESULTS: The best method to detect iron deficiency as estimated by the Youden index was the Thomas-Plot method. This method was very specific in cases without evidence of inflammatory processes (CRP <6 mg/L) and very sensitive in cases with elevated CRP (>6 mg/L). The sTfR-Ferritin-index came second and sTfR was the best single blood test to detect iron deficiency according to the Youden index. CONCLUSION: The Thomas-Plot method and the sTfR-Ferritin-index proved to be the most reliable blood tests to diagnose iron deficiency. These parameters can eliminate the need of using bone marrow aspirate to diagnose iron deficiency in some cases.