Serum transferrin receptor in hereditary hemochromatosis and African siderosis

Zinc protoporphyrin, a useful parameter to address hyperferritinemia

Zinc protoporphyrin (ZPP) is produced instead of heme as soon as iron support to erythropoiesis becomes insufficient. In iron deficiency the intra-erythrocytic ZPP concentration is increased. The aim of this study was to investigate whether ZPP is influenced by increased iron levels in hereditary hemochromatosis (HE) and is useful in the clarification of hyperferritinemia. Twenty HE patients and 160 patients with hyperferritinemic caused by anemia of chronic disorders, liver diseases, transfusional iron overload and hematologic or solid malignancies were enrolled. ZPP was measured using the Aviv front-face hematofluorometer (normal <or= 40 micromol/mol heme). In HE, ZPP was significantly lower (median, 20 micromol/mol heme; p = 0.0005) compared to our historical control group. At diagnosis, 15 (75%) HE patients had ZPP values <or=25 micromol/mol heme. After phlebotomy, ZPP remained unchanged (median, 23 micromol/mol heme), although the initially high ferritin concentration decreased to normal. ZPP values in the other hyperferritinemic groups were significantly higher compared to HE and control groups. In contrast to HE, ZPP values <or=25 micromol/mol heme were only observed in 11% of cases with non-transfusional hyperferritinemia. The diagnostic accuracy of a ZPP <or=25 micromol/mol heme to detect HE in non-transfused hyperferritinemic patients was 87%, with a sensitivity of 75% and a specificity of 89%. Showing significantly lower values in HE, ZPP seems to be a useful parameter in distinguishing HE from other hyperferritinemic disorders as those conditions are generally accompanied by an increased ZPP.

Iron assessment tests: transferrin receptor vis-à-vis zinc protoporphyrin

OBJECTIVES

To review and compare the biochemical, analytical, and clinical features of two relatively new tests for assessing iron status and diagnosing iron disorders, namely, the serum transferrin receptor concentration (sTfR) and the erythrocyte zinc protoporphyrin/heme ratio (ZnPP/H). To consider the merits of each test for the diagnosis of iron disorders with emphasis on iron-deficient erythropoiesis, especially in the clinically important preanemia stage of iron depletion.

CONCLUSIONS

Although the basic biochemical mechanisms underlying the two tests are very different, both of these tests are noteworthy because they are considered to reflect iron status in the bone marrow. The principal advantage to serum transferrin receptor is the lack of a response to anemia of chronic disease (ACD), which affects other iron status indicators, for example, ferritin and transferrin saturation. The principal advantage to erythrocyte zinc protoporphyrin is low cost, but point-of-care testing and simplicity can also be advantages. Both serum transferrin receptor and erythrocyte zinc protoporphyrin have been demonstrated to be useful in a variety of clinical situations. Serum transferrin receptor can be best used in diagnosing iron disorders, especially for patients with pathologies that may affect iron metabolism. Erythrocyte zinc protoporphyrin can be best used as a primary screening test for assessing iron status, especially in patients likely to have uncomplicated iron deficiency. While these applications are primary, both tests are more broadly useful, for example, in monitoring iron therapy.

Soluble transferrin receptor for the evaluation of erythropoiesis and iron status

Iron transport in the plasma is carried out by transferrin, which donates iron to cells through its interaction with a specific membrane receptor, the transferrin receptor (TfR). A soluble form of the TfR (sTfR) has been identified in animal and human serum. Soluble TfR is a truncated monomer of tissue receptor, lacking its first 100 amino acids, which circulates in the form of a complex of transferrin and its receptor. The erythroblasts rather than reticulocytes are the main source of serum sTfR. Serum sTfR levels average 5.0+/-1.0 mg/l in normal subjects but the various commercial assays give disparate values because of the lack of an international standard. The most important determinant of sTfR levels appears to be marrow erythropoietic activity which can cause variations up to 8 times below and up to 20 times above average normal values. Soluble TfR levels are decreased in situations characterized by diminished erythropoietic activity, and are increased when erythropoiesis is stimulated by hemolysis or ineffective erythropoiesis. Measurements of sTfR are very helpful to investigate the pathophysiology of anemia, quantitatively evaluating the absolute rate of erythropoiesis and the adequacy of marrow proliferative capacity for any given degree of anemia, and to monitor the erythropoietic response to various forms of therapy, in particular allowing to predict response early when changes in hemoglobin are not yet apparent. Iron status also influences sTfR levels, which are considerably elevated in iron deficiency anemia but remain normal in the anemia of inflammation, and thus may be of considerable help in the differential diagnosis of microcytic anemia. This is particularly useful to identify concomitant iron deficiency in a patient with inflammation because ferritin values are then generally normal. Elevated sTfR levels are also the characteristic feature of functional iron deficiency, a situation defined by tissue iron deficiency despite adequate iron stores. The sTfR/ferritin ratio can thus describe iron availability over a wide range of iron stores. With the exception of chronic lymphocytic leukemia (CLL) and high-grade non-Hodgkin's lymphoma and possibly hepatocellular carcinoma, sTfR levels are not increased in patients with malignancies. We conclude that soluble TfR represents a valuable quantitative assay of marrow erythropoietic activity as well as a marker of tissue iron deficiency.

Peripheral blood erythrocyte parameters in hemochromatosis: evidence for increased erythrocyte hemoglobin content

We studied peripheral blood erythrocyte parameters and HFE genotypes in 94 hemochromatosis probands and 132 white, normal control subjects. Mean red blood cell counts in probands and control subjects were not significantly different. However, mean values of hemoglobin, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) were significantly higher in C282Y/C282Y probands (n = 60) than in wild-type control subjects (n = 65). Probands with other HFE genotypes also had increased mean erythrocyte parameters (other than red blood cell count). Peripheral blood smears prepared before therapeutic phlebotomy revealed that erythrocytes in many probands had increased diameters and were well filled with hemoglobin. Erythrocyte parameters were similar in C282Y/C282Y probands with and without hepatomegaly, elevated serum concentrations of hepatic enzymes, hepatic cirrhosis, diabetes mellitus, arthropathy, or hypogonadism. Among C282Y/C282Y probands, significantly greater values of MCV (but not other erythrocyte parameters) occurred among those who had transferrin saturation values of 75% or greater or iron overload at diagnosis. After iron depletion, the mean MCV, MCH, and MCHC values of C282Y/C282Y probands decreased but remained significantly greater than values in wild-type control subjects. Mean values of prephlebotomy MCH and MCHC concentrations were lower in HLA-A3-positive than in HLA-A3-negative C282Y/C282Y probands. We conclude that increased values of mean hemoglobin, hematocrit, MCV, MCH, and MCHC in hemochromatosis probands are caused primarily by increased iron uptake and hemoglobin synthesis by immature erythroid cells. Mechanisms of iron uptake by erythrocytes that could explain these results are discussed.

Increased Serum Transferrin Saturation Is Associated with Lower Serum Transferrin Receptor Concentration

Background: Serum transferrin receptor (sTfR) concentrations are increased in iron deficiency. We wished to examine whether they are decreased in the presence of potential iron-loading conditions, as reflected by increased transferrin saturation (TS) on a single occasion.

Methods: We compared sTfR concentrations between 570 controls with normal iron status and 189 cases with increased serum TS on a single occasion; these latter individuals may be potential cases of iron overload. Cases and controls were selected from adults who had been examined in the third National Health and Nutrition Examination Survey (1988–1994) and for whom excess sera were available to perform sTfR measurements after the survey’s completion. Increased TS was defined as &gt;60% for men and &gt;55% for women; normal iron status was defined as having no evidence of iron deficiency, iron overload, or inflammation indicated by serum ferritin, TS, erythrocyte protoporphyrin, and C-reactive protein.

Results: Mean sTfR and mean log sTfR:ferritin were ∼10% and 24% lower, respectively, in cases than in controls (P &lt;0.002). Cases were significantly more likely to have an sTfR value &lt;2.9 mg/L, the lower limit of the reference interval, than were controls (odds ratio = 1.8; 95% confidence interval, 1.04–2.37).

Conclusion: Our results support previous studies that suggested that sTfR may be useful for assessing high iron status in populations.

Soluble transferrin receptor in sickle cell diseases: correlation with spleen function

OBJECTIVE

To correlate spleen function with soluble transferrin receptor (sTfR) levels and red cell ferritin (RCF) values in patients with sickle cell diseases.

DESIGN

Prospective study.

LOCATION

University Hospital, School of Medical Sciences, State University of Campinas; a tertiary hospital.

PARTICIPANTS

60 patients with sickle cell diseases, in a steady state, who had not received blood transfusions for 3 months; 28 normal individuals with no clinical or laboratory signs of anemia.

MEASUREMENTS

Determination of serum iron, transferrin iron-binding capacity, serum ferritin, RCF and sTfR. Evaluation of spleen function: erythrocytes with pits were quantified.

RESULTS

Patients with sickle cell anemia had sTfR levels significantly higher than in normal individuals or those with HbSC (p=0.0001) and there was an inverse correlation between sTfR and fetal Hb (p=0.0016). RCF values were significantly higher in sickle cell anemia patients than in normal individuals or those with HbSC (p=0.0001), and there was a correlation between RCF and pitted erythrocytes (p=0.0512).

CONCLUSION

The association between sTfR and fetal Hb confirms the contribution of fetal Hb to improving the hemolytic state by minimizing the consequent reactive erythrocyte expansion. High sTfR levels are not related to the degree of spleen function deficiency seen in sickle cell disease patients. The deficiency in the exocytosis process of the spleen occurring in sickle cell anemia patients may contribute to their accumulation of RCF.

Serum transferrin receptors are decreased in the presence of iron overload

To test the hypothesis that the quantities of circulating transferrin receptors are reduced in iron overload, we studied serum transferrin receptors and indirect measures of iron status in 150 subjects from rural Zimbabwe. We found significant inverse correlations between serum concentrations of transferrin receptors and ferritin, the ratio of ferritin to aspartate aminotransferase, and transferrin saturation (r ≥0.44; P &lt;0.001). The mean ± SD concentration of serum transferrin receptors in 23 subjects classified as having iron overload (ferritin &gt;300 μg/L and transferrin saturation &gt;60%) was 1.55 ± 0.61 mg/L, significantly lower than the 2.50 ± 0.62 mg/L in 75 subjects with normal iron stores (ferritin 20–300 μg/L and transferrin saturation 15–55%; P &lt;0.0005) and the 2.83 ± 1.14 mg/L in 8 subjects with iron deficiency (ferritin &lt;20 μg/L; P = 0.001). In keeping with the regulation of transferrin receptor expression at the cellular level, our findings suggest that serum transferrin receptors are decreased in the presence of iron overload.