This study investigated the efficiency of serum soluble transferrin receptor (sTfR) for assessing body iron status at different stages of iron deficiency. Among 72 patients with advanced iron-deficiency anemia (IDA), the sensitivity and specificity of sTfR (at a diagnostic cutoff of 3.24 mg/L) were 70.8% and 90.6%, respectively, with a positive predictive value of 85.0%. Sensitivities of sTfR in patients at the earliest stage of iron deficiency (n=41) and the intermediate stage of iron-deficient erythropoiesis (n=15) were 21.9% and 26.7%, respectively, at the same cutoff value of sTfR. Serum ferritin concentrations averaged 6.7+/-1.9 microg/L in IDA patients with sTfR <3.24 mg/L, which were significantly above the values in IDA patients with sTfR >or=3.24 mg/L (4.8+/-1.2 microg/L, p<0.05). In healthy controls, blood reticulocyte counts were significantly higher in subjects with sTfR >or=3.24 mg/L than in those with sTfR <3.24 mg/L (0.045+/- 0.013 (10(12)/L) vs 0.034+/- 0.011 (10(12)/L), p<0.05]. In conclusion, sTfR level is not a sensitive indicator for the early or intermediate stages of iron deficiency, although sTfR assay can be a useful aid in the diagnosis of advanced IDA. Serum sTfR concentration has significant relationships with blood reticulocyte counts in healthy subjects and with serum ferritin levels in IDA patients.