Circulating transferrin receptor assay--coming of age.

Transferrin receptors are transmembrane proteins present on the surface of most cells. Under normal circumstances, the iron required for cellular metabolism is acquired via transferrin receptors. Several recent reviews detail the methods of iron acquisition, the intracellular throughput of transferrin receptors, and the controlling mechanisms in the cell‘s quest to acquire and store iron [1–3]. The cells of different organ systems show considerable differences in the concentration of cellular transferrin receptor, the highest concentrations being found in cells of organs with the highest iron requirements, such as the erythroid bone marrow and placenta [4]. The concentration of cell surface transferrin receptor is carefully regulated by transferrin receptor mRNA according to the internal iron content of the cell and its individual iron requirements. Iron-deficient cells contain increased numbers of receptor, while receptor numbers are downregulated in iron-replete cells [4, 5]. Transferrin receptor was identified in serum [6] after investigators recognized that the molecule was secreted into culture media in reticulocyte and erythroleukemia cell models [7, 8], and the concentrations of secreted receptor were found to correlate with the total receptor content of the cells. Subsequent studies showed that serum concentrations of transferrin receptor increase in iron-deficiency anemia, making it a useful marker in the diagnosis of microcytic anemias [9]. Circulating transferrin receptor is a truncated form of tissue receptor [10], produced by proteolytic cleavage of cellular receptor, and for the most part circulates attached to transferrin [11, 12]. The reference interval for concentration of circulating receptor varies between different assay systems, depending on the choice of calibrators. In this issue of Clinical Chemistry, the transferrin receptor assay recently approved by the US Food and Drug Administration is described by Allen et al. [13]. The calibrators used in this enzyme immunoassay system are purified transferrin receptors isolated from serum, and the assay displays a reference interval of 0.57–2.8 mg/L. A second assay system, described in a previous issue by Suominen et al. [14], has a reference interval of 1.3–3.3 mg/L; the nature of the calibrators used in this assay is not specified. Other previously described assay systems, utilizing transferrinbound transferrin receptor calibrators isolated from intact placental transferrin receptor, report reference intervals for serum transferrin receptor of ;3.5–8.5 mg/L [15, 16]. The calibrators utilized in these latter assay systems may more closely simulate the in vivo circumstance of circulating transferrin receptor, which ordinarily circulates attached to transferrin and is not found in appreciable amounts in the unbound state. It is hoped that collaborative efforts will be made to establish optimal calibrators that can be made available to all users of transferrin receptor assays, as has been achieved with other proteins, e.g., ferritin [17]. Circulating transferrin receptor concentrations do not differ between healthy males and females [15, 16]. Allen et al. [13], however, have found that concentrations are slightly higher in blacks than nonblacks. They also report that the concentrations vary in populations living at different altitudes, the higher concentrations occurring at higher altitudes. These differences make it necessary for individual laboratories to establish their own reference values. The effect of altitude on transferrin receptor concentrations reflects differences in bone marrow erythroid activity. Various studies, including ferrokinetic data, indicate that circulating receptor concentrations vary according to the intensity of erythropoiesis [16, 18], with erythroid bone marrow precursors accounting for at least two-thirds of circulating receptor concentrations. Transferrin receptor concentrations are decreased in disorders with reduced erythropoiesis such as aplastic anemia or after bone marrow ablation for hematopoietic stem cell transplantation [15, 16, 19]. The concentrations may also be reduced in iron-overload disorders, as described by Khumalo et al. [20] in the current issue of Clinical Chemistry, reflecting the overall decreased transferrin receptor production at the cellular level. Transferrin receptor concentrations are increased in disorders with expanded erythropoiesis, such as hemolytic anemias [15, 16, 18], and in conditions associated with ineffective erythropoiesis, such as myelodysplastic syndromes and megaloblastic anemias [21, 22]. The main value of the transferrin receptor assay is in the differential diagnosis of microcytic anemias [4, 23]. Circulating transferrin receptor concentrations increase in tissue iron deficiency [15, 24, 25], reflecting the degree of iron deficiency in the erythroid precursors of the marrow. In a study in which iron-deficiency anemia was gradually induced in generally healthy subjects by serial phlebotomy, serum ferritin declined as iron stores decreased, and circulating transferrin receptor concentrations increased only once the iron stores were depleted, before changes in the other accepted markers of tissue iron deficiency, e.g., transferrin saturation, mean red cell volume, and erythrocyte protoporphyrin concentrations [26]. By the time that anemia developed, none of these accepted markers had appreciably deviated from their “normal” reference intervals. In summary, when iron stores decline, serum ferritin concentrations drop until iron stores are depleted, at which time the ferritin concentration falls below the lower limit of the reference interval. With further iron loss, and as iron-deficient erythropoiesis begins, circulating transferrin receptor concentrations begin to increase and continue to do so as the severity of iron-deficient erythropoiesis increases, reflecting the increasing numbers of receptors on the erythroid cells of the bone marrow. The ratio of transferrin receptor to ferritin displays an inverse linear relationship to iron status, covering the spectrum from usual iron stores in health to substantial functional iron deficiency [25, 26]. The adequate investigation of microcytic anemia includes measurement of serum ferritin. Assay of transferrin receptor Editoral