Unsaturated Iron-Binding Capacity (UIBC): This test measures how much iron your blood could carry but isn’t currently carrying. Think of it as the “empty seats” on the iron-carrying protein called transferrin. If your UIBC is high, it usually means there’s not enough iron in your blood (iron deficiency), because there are more empty spots available. If your UIBC is low, it could mean there’s too much iron, or your body isn’t making enough transferrin.