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. It is a cytosolic molecule comprised of subunits that self-assemble into a nanocage capable of containing up to 4500 iron atoms. 
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.
Iron deficiency anemia
The serum ferritin is the best laboratory test for storage of marrow iron. However, the accuracy of the ferritin has not been studied since the World Health Organization used a recombinant ferritin preparation to create a third, traceable International Standard in 1997.
|Test||Cutoff value||Likelihood ratio|
|Ferritin||≥ 100 ng/ml||0.08|
|Ferritin||< 25 ng/ml||< 8.83|
|A likelihood ratio > 10 helps establish a diagnosis while a ratio < 0.1 helps exclude a diagnosis.|
Other causes of high ferritin levels
Infections and neoplasms can elevate the ferritin over 1,000 ng/mL.
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