Soluble transferrin receptor and sTfR-F index for assessing concurrent iron deficiency in patients with chronic renal failure

Abstract

Dear Editor, We read with interest the comments of Dr. Koulaouzidis [1] regarding the results of our study, which was recently reported on the usefulness of the combination index of serum-soluble transferrin receptor (sTfR) and mean corpuscular hemoglobin (MCH) to differentiate coexisting iron deficiency in patients with end-stage renal disease (ESRD) [2]. First, Dr. Koulaouzidis raised a question as to why the sTfR value was used instead of the sTfR-F index (the ratio of sTfR to log ferritin) to evaluate iron deficiency in patients with chronic renal failure. In fact, the sTfR-F index has been suggested as a good estimate of body iron status [3]. Several investigators reported that the sTfR-F index is an additional biochemical marker for identification of irondeficient erythropoiesis [4]. In our study, however, the sTfR-F index was not superior to sTfR value in the correlation coefficients with serum iron (r=−0.22 vs. r=−0.24) and transferrin saturation (r=−0.23 vs. r=−0.27) in the patients with ESRD. The sTfR-F index may reflect the erythropoietic activity, but it seems to depend on the body iron status of subject populations. The relationships between the sTfR-F index and erythropoietic parameters are fairly different from each other between healthy individuals and iron-deficient anemic subjects [5]. In the preliminary test of our study, similar findings were also observed in the patients with rheumatoid arthritis. On the basis of these results, we selected the sTfR value rather than sTfR-F index as an indicator of iron status in the patients with ESRD. Secondly, Dr. Koulaouzidis made a comment on the low cut-off level of sTfR, which was used to diagnose iron deficiency in our study, and emphasized the importance of the normal reference value of sTfR that was obtained from the healthy controls. However, it is likely that there are large differences in sTfR production between healthy subjects and the patients with ESRD, especially under the condition of suppressed erythropoiesis. It is hard to apply the reference intervals of healthy subjects for the patients with chronic renal failure. In our study, mean values of sTfR in the patients with ESRD (1.50±0.97 mg/l) were significantly lower than those in age-matched healthy controls (2.13±0.51 mg/l, p<0.05) [6]. The reason for the low limit of sTfR seems to be that the cut-off point was determined on the basis of the mean sTfR concentrations from the patients with chronic renal failure. Another possible explanation is that the cutoff value of sTfR was defined as a combination index, in conjunction with MCH level in a small number of patients. In healthy subjects, the reference range of serum sTfR concentration shows variances according to investigators. It may reflect the difference of the healthy controls enrolled in the corresponding study. In our experience, fourfold or more Ann Hematol (2008) 87:575–576 DOI 10.1007/s00277-008-0460-5