Iron-Binding Capacity

The index of transferrin present in circulating blood is the iron-binding capacity. Transferrin can be nearly one-third saturated with iron. The unsaturated iron-binding capacity (UIBC) is measured using radioactive iron or spectrophotometric approaches. The sum of the UIBC and the plasma iron is the total iron-binding capacity (TIBC). ^[1] Direct measurement of the TIBC may also be performed. ^[2]

The iron-binding capacity reference range is 255-450 μg/dL.

Increases in iron-binding capacity are observed with the following:

• Iron deficiency states

• Acute liver damage

• Acute and chronic blood loss

• Late pregnancy

• Progesterone birth control pills

Decreases in iron-binding capacity are associated with the following:

• Hemochromatosis

• Hemosiderosis

• Thalassemia

• Hyperthyroidism

• Nephrotic syndrome

• Anemia of chronic diseases ^[3]

Container - Tiger-top or red tube ^[4]

Panels - Serum iron panel

Description

The measurement of the blood’s capacity to bind iron with transferrin is the iron-binding capacity. Determining total iron-binding capacity (TIBC) is an indirect means of assessing transferrin levels.

However, although TIBC correlates with the serum transferrin level, the relationship is not linear over a wide range of transferrin levels. TIBC must not be confused with unsaturated binding capacity (UIBC), which represents the residual after subtraction of serum iron from TIBC. The relationship of TIBC and transferrin is disrupted in individuals with conditions that affect transferrin-binding capacity or other iron-binding proteins.

Iron overload conditions (eg, iron poisoning, hemolytic anemia, sideroblastic anemia, thalassemia, hemochromatosis, pyridoxine deficiency, aplastic anemia) are associated with an increased percentage of transferrin saturation with iron.

Iron deficiency is associated with a decreased percentage of transferring saturation, usually less than 16%.

Transferrin levels may be used in the assessment of nutritional status.

Test results are influenced by recent transfusions. ^{[5, 6]}

A decrease in TIBC of more than 20 mg/dL over 6 months was associated with a death hazard ratio of 1.57 (95% CI: 1.04-2.36; P = 0.03) compared with the stable TIBC group, in a study by Bross et al. ^[7] Thus, a low baseline serum TIBC is associated with iron deficiency, protein emerge malnutrition, inflammation, a poor quality of life, and increased risk of mortality. A decline in TIBC is independently associated with an increased mortality risk.

A study by Ikeda-Taniguchi et al indicated that in patients undergoing maintenance hemodialysis, a significant association exists between TIBC in the lower tertiles and reductions in muscle mass and albumin. ^[8]

Indications/Applications

Iron-binding capacity is used in the following:

• Diagnosis of anemia

• Screening for iron overload, acute hepatitis, and late pregnancy

• Panel for serum iron to find out the percentage of saturation for the diagnosis of iron deficiency

Considerations

The index of transferrin present in circulating blood is the iron-binding capacity. Transferrin can be nearly one-third saturated with iron. The unsaturated iron-binding capacity (UIBC) is measured using radioactive iron or spectrophotometric approaches. The sum of the UIBC and the plasma iron is the total iron-binding capacity (TIBC). ^[1] Direct measurement of total iron-binding capacity may also be performed.

The TIBC is increased by estrogen and oral contraceptives. Decreases in the TIBC may be caused by asparaginase, chloramphenicol, corticotrophin, cortisone, and testosterone.

The UIBC and TIBC are usually increased in patients with iron-deficiency anemia, in whom a transferrin saturation of 15% or less is noted. Exceptions to this rule detract considerably from the diagnostic value of measuring transferrin saturation in patients with suspected iron deficiency. ^{[1, 9]} A low serum iron concentration is typically present along with a normal transferrin saturation in individuals with anemia of chronic inflammation. ^[1]