TIBC (total iron-binding capacity): what the result shows

Iron travels around the blood attached to a protein called transferrin, made by the liver. Lab Tests Online UK says TIBC is a measure of all the proteins in the blood available to bind with iron, including transferrin, and that it is a good indirect measurement of transferrin because transferrin is the primary iron-binding protein. The body makes transferrin in relation to the need for iron: when stores are low, transferrin rises and vice versa.

A related test, UIBC (unbound iron-binding capacity), measures the part of the capacity that is not yet occupied by iron. TIBC equals the iron already bound plus the UIBC. In healthy people about one third of the binding sites on transferrin are in use, and that fraction is the transferrin saturation.

TIBC or transferrin is used with iron and ferritin to evaluate people suspected of having too much or too little iron. It may be requested when you have anaemia with small, pale red cells, when a full blood count is abnormal, or when haemochromatosis (too much stored iron) is a concern, for example with a family history. Symptoms prompting iron tests include chronic tiredness, breathlessness, dizziness, weakness, headaches, leg pains and palpitations.

The NHS notes that iron deficiency anaemia is usually checked with a full blood count and that if the cause is unclear the GP may arrange other tests, including a faecal immunochemical test, or refer you to a specialist.

Lab Tests Online UK says you may be instructed to fast for 12 hours before the test, drinking only water. Iron tablets should not be taken for 24 hours beforehand because iron is absorbed rapidly and can make blood iron levels falsely high. Blood is taken from a vein in the arm.

Taking the oral contraceptive pill can raise the TIBC, and chloramphenicol can lower it. Some laboratories routinely use only red cell studies and ferritin to investigate anaemia and monitor treatment, so you may not always be offered TIBC.

A high TIBC or transferrin, or a low transferrin saturation, usually indicates iron deficiency. A low TIBC or transferrin, or a high saturation, may occur in haemochromatosis. Other causes of abnormal results include anaemia caused by infections and chronic diseases, malnutrition, cirrhosis and nephrotic syndrome, a kidney disease that causes protein loss in urine. Because the liver makes transferrin, TIBC is also low in liver disease.

The table below reproduces the pattern summary given by Lab Tests Online UK for different states of iron.

TIBC is rarely the deciding test. Serum iron varies through the day and from day to day, so it is not a useful measure of iron status alone, and ferritin is now recommended instead of serum iron to aid diagnosis of iron deficiency anaemia. Ferritin can however be raised by inflammation, so TIBC and saturation help when the picture is mixed, such as iron deficiency together with a chronic condition.

If the pattern indicates iron deficiency the next step is to find the cause, such as heavy periods, diet, pregnancy or bleeding from the digestive tract, and then to replace iron. The NHS says iron tablets are usually taken for about six months and that repeat blood tests check recovery. If the pattern suggests iron overload, HFE genetic testing may be offered to confirm haemochromatosis.

Do not start iron supplements before the cause is understood; the NHS warns that an overdose of iron in a young child can be fatal, so keep tablets out of reach of children.