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Along with other iron tests, to assess your body's ability to transport iron in the blood; to help diagnose iron-deficiency or iron overload
When you have low hemoglobin and hematocrit on a complete blood count (CBC); when your healthcare practitioner suspects you may have too much iron (overload) or too little iron (deficiency) in the body
A blood sample drawn from a vein in your arm
You may be instructed to have your blood drawn in the morning and/or fast for 12 hours before the test; in this case, only water is allowed. Follow any instructions from your healthcare practitioner and/or from the laboratory performing the test.
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 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 interpreted together to help diagnose and/or monitor iron deficiency or iron overload.
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:
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.
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 iron deficiency, iron deficiency anemia, or too much iron in the body (overload).
These tests may be ordered along with other iron tests when results from a routine complete blood count (CBC) show that a person's hemoglobin and hematocrit are low and their red blood cells are smaller and paler than normal (microcytic and hypochromic), suggesting iron deficiency anemia even though other clinical symptoms may not have developed yet.
Iron tests may be ordered when a person develops signs and symptoms of anemia such as:
Testing may be ordered when iron overload is suspected. Signs and symptoms of iron overload will vary from person to person and tend to worsen over time. They are due to iron accumulation in the blood and tissues. These may include:
Testing is also ordered when there is a case of suspected iron poisoning. This is most common in children who accidentally overdose with vitamins or other supplements containing iron.
The results of transferrin tests, TIBC, or UIBC are usually evaluated in conjunction with other iron tests. A summary of the changes in iron tests seen in various diseases of iron status is shown in the table below.
The early stage of iron deficiency is the slow depletion of iron stores. This means there is still enough iron to make red cells but the stores are being used up without adequate replacement. The serum iron level may be normal in this stage, but the ferritin level will be low.
As iron deficiency continues, all of the stored iron is used and the body tries to compensate by producing more transferrin to increase iron transport. The serum iron level continues to decrease and transferrin and TIBC and UIBC increase. As this stage progresses, fewer and smaller red blood cells are produced, eventually resulting in iron deficiency anemia. Transferrin saturation is decreased with iron deficiency.
If the iron level and transferrin saturation are high, the TIBC, UIBC and ferritin are normal and the person has a clinical history consistent with iron overdose, then it is likely that the person has iron poisoning. Iron poisoning occurs when a large dose of iron is taken all at once (acute) or over a long period of time (chronic). Iron poisoning in children is almost always acute, occurring in children who ingest their parents' iron supplements. In some cases, acute iron poisoning can be fatal.
A person who has mutations in the HFE gene is diagnosed with hereditary hemochromatosis. However, while many people who have hemochromatosis will have no symptoms for their entire life, others will start to develop symptoms such as joint pain, abdominal pain, and weakness in their 30's or 40's. Men are affected more often than women because women lose blood during their reproductive years through menstruation.
Iron overload may also occur in people who have hemosiderosis and in those who have had repeated transfusions. This may occur with sickle cell anemia, thalassemia major, or other forms of anemia. The iron from each transfused unit of blood stays in the body, eventually causing a large buildup in the tissues. Some persons with alcoholism and with chronic liver disease also develop iron overload.
Recent blood transfusions can affect test results as can iron injections or transfused iron. Multiple blood transfusions can sometimes lead to iron overload.
A high TIBC, UIBC, or transferrin usually indicates iron deficiency, but they are also increased in pregnancy and with the use of oral contraceptives.
A low TIBC, UIBC, or transferrin may also occur if someone has malnutrition, inflammation, liver disease, or nephrotic syndrome. However, the tests are usually not used to assess these conditions.
The calculation is:
Transferrin saturation (%) = (Serum iron level x 100%) / TIBC
TIBC 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.
In healthy individuals, transferrin is one-third saturated with iron. This means that there is about two-thirds held in reserve. In iron deficiency, all of the stored iron is used and the body tries to compensate by producing more transferrin to increase iron transport. While the serum iron level continues to decrease, the transferrin level increases. Thus, the amount of transferrin available to bind iron (TIBC) increases and the amount of transferrin saturated with iron (i.e., percent transferrin saturation) decreases.
Transferrin is a protein that may decrease during any inflammatory process and is referred to as a negative acute phase reactant. Chronic inflammation, infections, and malignancies may cause changes in transferrin levels.
Yes, there are numerous causes of anemia. However, iron deficiency is one of the most common. If iron tests rule out iron deficiency, another source for the anemia must be found. See the article on Anemia for more on this.
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