Total iron binding capacityTIBC
How much iron your blood could carry if every transport site were filled.
Total iron binding capacity measures the blood's maximum capacity to bind iron, which in practice is a measure of how much transferrin is available. It is the empty-seat count on the transport system.
It matters because it moves in the opposite direction to iron stores in the commonest situation: in iron deficiency the body makes more transferrin, so capacity rises while serum iron falls. That divergence is more informative than either number alone.
In practice TIBC is combined with serum iron to calculate transferrin saturation, which is the figure most often used to judge iron availability.