Iron binding capacity is of two types, TIBC and unsaturated iron-binding capacity (UIBC). When iron stores are depleted, the transferrin levels increase in the blood. As only one-third of transferrin is saturated with iron, so the transferrin present in serum has an extra binding capacity (67%). This is unsaturated iron-binding capacity.
UIBC is used for the calculation of TIBC. TIBC = UIBC + iron