Serum iron is bound to transferrin, but only about one third of the iron binding sites are saturated with iron. The unsaturated iron-binding capacity of transferrin (UIBC) denotes the available iron-binding sites of serum. The amount of iron that serum transferrin can bind when completely saturated with an excess of Fe+3 is the total iron-binding capacity (TIBC).
The method measures the TIBC by first saturating the transferrin with escess of Fe+3. The remaining iron is adsorbed with magnesium carbonate, and once the binding process is complete the quelator is removed by centrifugation, and an assay for iron content performed in the supernatant. From this measurement the TIBC value is obtained.
When the serum iron (SI) determination is performed concurrently with the TIBC and the result substracted from the TIBC value, the difference yields the unsaturated iron-binding capacity (UIBC), or seric transferrin not bound to iron.