The uptake of iron by cells is mediated by a cell-surface transferrin receptor (TfR). The amount of transferrin receptor expressed on a cell is proportional to the cells need for iron. A proteolytic product of the transferrin receptor circulates in plasma as soluble transferrin receptor (sTfR). The soluble transferrin receptor (sTfR) has been introduced as a promising new diagnostic tool for differentiating between iron deficiency anemia (IDA) and anemia of chronic disease (ACD). This test is useful in the diagnosis of iron deficiency associated with inflammation or infection, the diagnosis of functional iron deficiency in children, adolescents and athletes (in whom iron reserves and therefore ferritin levels are unreliable), the early diagnosis of iron-deficient erythropoiesis (e.g. in pregnant women), the differential diagnosis of anemia, estimating the correct dose of erythropoietin (EPO) in patients with impaired kidney function, and to screen for EPO abuse. Serum soluble transferrin receptor INCREASES in iron deficiency and is usually unaffected by chronic disease states.