What is Total Iron Binding Capacity?
Also Known As:
Iron Binding Capacity, IBC, Serum Iron-Binding Capacity, Siderophilin, TIBC, UIBC
Transferrin, Total Iron Binding Capacity, Unsaturated Iron Binding Capacity, Transferrin Saturation
Transferrin is the main protein in the blood that binds to iron and transports it throughout the body. A transferrin test directly measures the level in the blood. Alternatively, transferrin may be measured indirectly (or converted by calculation) so that its level is expressed as the amount of iron it is capable of a binding. This is called the total iron-binding capacity (TIBC).
Iron is an essential nutrient that, among other functions, is necessary for the production of healthy red blood cells (RBCs). It is a critical part of hemoglobin, the protein in RBCs that binds oxygen in the lungs and releases oxygen as blood circulates to other parts of the body. The body cannot produce iron and must absorb it from the foods we eat or from supplements.
Normally, iron is transported throughout the body by transferrin, which is produced by the liver. In healthy people, most iron is incorporated into the hemoglobin within RBCs. The remainder is stored in the tissues as ferritin or hemosiderin, with additional small amounts used for other purposes (e.g., to produce other proteins such as myoglobin and some enzymes).
The transferrin test, TIBC, UIBC, and transferrin saturation, along with other iron tests, help evaluate the amount of iron in the body by measuring several substances in the blood. These tests are often ordered at the same time and the results are interpreted together to help diagnose and/or monitor iron deficiency or iron overload.
- Serum iron test—measures the total amount of iron in the liquid portion of the blood, nearly all of which is bound to transferrin.
- Transferrin test—directly measures the level of transferrin in the blood. The level depends upon liver function and a person’s nutritional status. Transferrin is a protein that may decrease during any inflammatory process and is referred to as a negative acute-phase reactant.
- TIBC (total iron-binding capacity)—measures the total amount of iron that can be bound by proteins in the blood. Since transferrin is the primary iron-binding protein, the TIBC test is a good indirect measurement of transferrin availability—the amount of transferrin that is available to bind to iron. (Note: Though TIBC is a reflection of the amount of transferrin available, TIBC and transferrin are not synonymous.)
- UIBC (unsaturated iron-binding capacity)—this test determines the reserve capacity of transferrin, i.e., the portion of transferrin that has not yet been saturated with iron.
- Transferrin saturation— dividing the iron concentration by the TIBC produces an estimate of how many of transferrin iron-binding sites are occupied; this is called the transferrin saturation. Under normal conditions, transferrin is typically one-third saturated with iron. This means that about two-thirds of its capacity is held in reserve. (Less commonly, the iron concentration may be divided by the transferrin concentration, not the TIBC. This similar estimate is usually called the transferrin index.)
- Ferritin—measures the level of ferritin, a protein made by almost all cells in response to increased iron. The ferritin level reflects the total body iron. It will be low when there is iron deficiency and high when there is an excess of iron in the body.
When the level of iron is insufficient to meet the body’s needs, the level of iron in the blood drops and iron stores are depleted. This may occur because:
- There is an increased need for iron, for example during pregnancy or childhood, or due to a condition that causes chronic blood loss (e.g., peptic ulcer, colon cancer)
- Not enough iron is consumed (either foods or supplements)
- The body is unable to absorb iron from the foods eaten in conditions such as celiac disease
Insufficient levels of circulating and stored iron may eventually lead to iron deficiency anemia (decreased hemoglobin and hematocrit, smaller and paler red cells). In the early stage of iron deficiency, no physical effects are usually seen and the amount of iron stored may be significantly depleted before any signs or symptoms of iron deficiency develop. If a person is otherwise healthy and anemia develops over a long period of time, symptoms may not appear before the hemoglobin in the blood drops below the lower limit of normal.
However, as the iron deficiency progresses, symptoms eventually begin to appear. The most common symptoms of anemia include fatigue, weakness, dizziness, headaches, and pale skin. Read the article on Anemia to learn more.
Conversely, too much iron can be toxic to the body. Iron storage and ferritin levels increase when more iron is absorbed than the body needs. Absorbing too much iron over time can lead to the progressive buildup of iron compounds in organs and may eventually cause their dysfunction and failure. An example of this is hemochromatosis, a rare genetic disease in which the body absorbs and builds up too much iron, even on a normal diet. Additionally, iron overload can occur when a person undergoes repeated blood transfusions.
How is it used?
The transferrin, total iron-binding capacity (TIBC), or unsaturated iron-binding capacity (UIBC) test may be used along with other iron tests to assess the amount of iron circulating in the blood, the total capacity of the blood to transport iron, and the amount of stored iron in the body. Testing may also help differentiate various causes of anemia.
Iron tests are often ordered together, and the results of each can help identify an iron deficiency, iron deficiency anemia, or too much iron in the body (overload).