Iron saturation is a measure of the amount of iron circulating through the bloodstream in a biologically available form. Lab tests for iron saturation are run when anemia is suspected in a patient.
Iron is carried through the bloodstream by a molecule called transferritin. Iron saturation levels are found by comparing the total amount of iron in the blood serum to the amount of transferritin present in the blood. When most of the iron in the blood being tested is bonded to transferrin, iron saturation is said to be high. People with low levels of total iron and low iron saturation are anemic. Excessive amounts of iron in the body result in low iron saturation as well. This is because when iron levels are too high, there is not enough transferrin to carry all the iron, so the majority of it winds up in an unbonded form in the serum.