None of the routinely available tests reflects iron intake; rather, they assess body stores. Iron is not absorbed efficiently. Iron absorption is dependent on dietary factors such as food source and the ascorbic acid content of the meal.
About 60-65% of the total body iron is in hemoglobin. Because the iron pool is associated with hemoglobin, persons with a larger blood volume (and thus more hemoglobin) have larger stores of iron. Males have more hemoglobin than females.
Serum iron is largely bound to the beta-globulin transferrin. There are usually 4-6 milligrams (mg) of transferrin-bound iron in plasma with a capacity to bind 25-30 mg. Iron is passed from the reticuloendothelial cells to the plasma where 50% is turned over in one hour. Serum iron measurements are unstable and vary greatly within a person and throughout the day. Normal adult levels of serum iron range from 65-200 micrograms per deciliter (mcg/dL). Low iron levels that do not reflect low body stores can result from blood loss, infections, chronic illness, and malignancies. High serum iron levels are seen in cases of megaloblastic anemia, when inflow into the plasma is increased, and in cases of aplastic anemia, when outflow from the plasma is decreased.
Transferrin levels are normally 350-380 mg/dL, and increase with iron deficiency and pregnancy. Levels are decreased with chronic disease, protein deficiency, or hepatic disease.
Interpretation of iron and transferrin are based on both values and the percent saturation. Low serum iron and low iron-binding capacity are the best indicators of anemia of chronic inflammation.
Ferritin is the major storage form of iron in the liver, spleen, and bone marrow. Serum ferritin levels are thought to reflect body stores. Normal values range from 15-400 nanograms (ng) per milliliter in males and 10-200 ng/ml in females. Lower values are indicative of iron deficiency with anemia. Serum ferritin levels between 15 and 45 ng/ml may indicate a deficiency in patients with chronic inflammation. High levels are seen with hemochromatosis and transfusions. Serum ferritin is the best screening test because levels fall only with decreased iron stores.
Protoporphyrins cannot be utilized for the synthesis of heme during iron deficiency; thus they will be found in higher levels when a patient is iron- deficient. However, FEP levels rise with other disorders of heme synthesis and so are not specific for iron deficiency anemia.