Medical Health Encyclopedia

Serum iron

Why the test is performed:

This test is performed when iron deficiency is suspected.

About 65% of the iron in the body is found in hemoglobin (in red blood cells) and about 4% in myoglobin (in skeletal muscle). About 30% of the iron in the body is stored (as ferritin or hemosiderin) in the liver, bone marrow, and spleen. A small percentage of the body's iron is in transport between various parts of the body or is a component of proteins in cells throughout the body.

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The body efficiently conserves iron so that only about 1 mg (men and post-menopausal women) or 1.8 mg (premenopausal adult women) is lost per day in the urine or menstrual blood. Since only about 10-15% of the iron we eat in our food is absorbed, even under optimum circumstances, the recommended daily allowance for iron is 10 mg (men and post-menopausal women) and 18 mg (premenopausal adult women). Pregnancy greatly increases the need for iron, and iron deficiency is most common in women of reproductive age.

Serum iron, as measured in the laboratory, is really transferrin-associated ferric iron. Each transferrin molecule can carry 2 iron atoms. Normally about 30% of the available sites are filled. This is called the percent transferrin saturation. By completely saturating all the available binding sites, it is possible to measure the total iron binding capacity (TIBC). This is really a measure of the transferrin in the serum. TIBC and percent transferrin saturation are usually measured at the same time serum iron is measured.

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