Soluble transferrin receptor and immature reticulocytes are not useful for distinguishing iron-deficiency anemia from heterozygous beta-thalassemia

ABSTRACT Iron deficiency and heterozygous beta-thalassemia are important causes of hypochromic-microcytic anemia. Two laboratory parameters are suggested for the differentiation of such anemia. High-fluorescence reticulocyte counts and soluble transferrin receptor levels were determined in iron-deficiency anemia patients (n = 49) and heterozygous beta-thalassemia patients (n = 43). There was no significant difference in high-fluorescence reticulocyte and soluble transferrin receptor values between the two groups, but a correlation was observed between high-fluorescence reticulocytes and soluble transfer-rin receptors in iron-deficiency anemia, probably due to increased receptor synthesis as a response to decreased iron content in erythrocytes.


INTRODUCTION
The soluble transferrin receptor is a truncated form of tissue receptor that circulates in serum as a complex of transferrin and its receptor. 1Studies indicate that the concentration of soluble transferrin receptor rises with enhanced erythropoiesis and iron deficiency. 2 Immature reticulocytes are released from bone marrow in the event of enhanced erythropoietic activity.Transferrin receptors are present in reticulocyte membranes, but not in mature red cells.The younger the reticulocyte is, the greater the number of receptors per cell. 1 Thus, a correlation between the number of immature reticulocytes and the soluble transferrin receptor concentration is expected.

PATIENTS AND METHODS
We studied 43 patients with heterozygous beta-thalassemia and 49 patients with irondeficiency anemia in order to determine the high-fluorescence reticulocyte count and soluble transferrin receptor concentration in the two groups, and whether these parameters could be used to distinguish heterozygous beta-thalassemia from iron-deficiency anemia.Fifty-seven non-anemic subjects were used as a control group.
Iron-deficiency anemia patients showed serum ferritin levels of under 30 ng/ml for men and 12 ng/ml for women (considered the minimum normal ferritin levels in our laboratory).Patients with hypochromic-microcytic anemia, hemoglobin A2 level over 3.4% and normal serum ferritin were considered to be heterozygous beta-thalassemia cases.The reticulocyte count and percentage of highly immature reticulocytes were obtained using a Cell-Dyn 3500 (Abbott  USA) analyzer.Reticulocytes were identified by a non-fluorescent method using new methylene blue as the dye.Fractions of reticulocyte immaturity were calculated on the basis of absorption intensity, and they were classified as mature, partially mature and highly immature reticulocyte fractions.Soluble transferrin receptor concentrations were estimated by an immunoenzymatic technique (Quantikine-R & D Systems  USA).The Kruskal-Wallis test was used for comparing the variables between groups.The correlation between high-fluorescence reticulocytes and soluble transferrin receptor was calculated via the Spearman correlation coefficient test.We considered p values equal to or lower than 0.05 to be significant.The capacity of the variables to differentiate between iron-deficiency anemia and heterozygous beta-thalassemia was studied by means of the receiver operating characteristic (ROC) curve.

DISCUSSION
The synthesis of surface transferrin receptor is proportional to the iron requirement of the cell, as a response to insufficient supply of transferrin iron. 3In addition, soluble transferrin receptor provides an assessment of erythropoiesis status, because the increase in soluble transferrin receptor concentration is proportional to erythroid marrow expansion. 1ur results confirm these concepts, as irondeficiency anemia and heterozygous betathalassemia showed soluble transferrin receptor concentrations that were higher than in the control group.In iron-deficiency anemia patients, soluble transferrin receptor determinations were higher than in heterozygous beta-thalassemia, but an overlap between the two groups was observed.Gimferrer et al., 4 using the same discriminator value for serum ferritin as we did, observed high values for soluble transferrin receptor in heterozygous beta-thalassemia and heterozygous betathalassemia associated with iron-deficiency anemia patients.The authors concluded that soluble transferrin receptor was not useful in diagnosing the association of iron-deficiency anemia with heterozygous beta-thalassemia.
The correlation between high-fluorescence reticulocytes and soluble transferrin receptor in iron-deficiency anemia may be explained by a high concentration of transferrin receptor synthesized as response to decreased iron content in red blood cell progenitors.

CONCLUSION
We conclude that soluble transferrin receptor and high-fluorescence reticulocytes are not useful for distinguishing heterozygous beta-thalassemia from iron-deficiency anemia patients, although they provide interesting data concerning erythropoietic activity.