Serum iron is bound to transferrin, but only one third of its capacity is saturated. The unsaturated binding capacity of transferrin or residual binding capacity (RBC) is indicative of the availability of serum binding receptors. The amount of iron that serum transferrin can bind when fully saturated with excess Fe3+ is the total binding capacity (TFC). The method measures the TFC by first saturating the transferrin with excess Fe3+. The excess iron is adsorbed with magnesium carbonate and once the binding process is complete, it is removed by centrifugation and the iron in the supernatant is determined. The figure found corresponds to the CFT. When the determination of serum iron is carried out at the same time as the TFR and the result is subtracted from the TFR value, the difference gives the free binding capacity (FBC), or serum transferrin not bound to iron.
