Ferritin

Ferritin is a water-soluble complex of protein and iron, which circulates in free form in the blood and can be measured in simple samples of serum or plasma. Circulating ferritin is in equilibrium to ferritin molecules contained in histiocytes of the less easily measured bone marrow.

In the histiocytes, the ferritin molecules accumulate into large cytoplasmic inclusions of hemosiderin, which is visible under the light microscope. Hemosiderin stains strongly with the Prussian Blue stain of marrow, and, if in sufficient concentration, is visible unstained.

The serum ferritin is the best laboratory test for storage of marrow iron. Other test that have been used are serum iron level, and serum transferrin level. While serum iron varies greatly intra-individually also in response to iron intake, the other two parameters mentioned change in an acute phase reaction (ferritin rises and transferrin false) and thus cannot reliably detect iron deficiency in the presence of inflammation. They also do not measure if the iron is actually available for hematopoiesis. Modern tests that circumvent this problem include soluble transferrin receptor (sTfr), transferrin saturation (TfS or TSAT), the hemoglobin content of reticulocytes or the percentage of hypochromic cells. Most of these can today be readily determined on automated laboratory analysis systems.