Transferrin is a protein present in blood that binds to iron. This bound complex then binds to Soluble Transferrin Receptors (sTFRs) on cells. When the number of complexes decrease, the number of vacant sTFRs increase. As a result, these sTFRs are released into the blood stream. Now, the number of the sTFRs present in the blood stream can indicate deficiency of iron in the system. Deficiency of iron, usually caused by lack of iron in the diet causes anaemia. This anaemia can either be due to above mentioned deficiency, or because of chronic illness. The sTFR Assay is used to identify whether an anaemia affected patient suffers from deficiency or some other chronic illness.