Total iron-bining capacity is a medical laboratory test which measures the extent to which iron-binding sites in the serum can be saturated. Of the large amount of iron that is stored in the body, only a small fraction is actually transported in the serum. The protein that does this transporting of iron around the body is called transferrin. A single molecule of transferrin can carry 2 molecules of iron, and in a normal person, only about 30% of these spots for iron are filled. Generally speaking, if the iron stores in the body are low, fewer of these spots on the transferrin molecules will be filled by iron, and the TIBC will therefore be higher.