Transferrin

What is being tested?
Transferrin is a glycoprotein made mainly in the liver and is the main iron transport protein; it also regulates iron absorption in the intestine. The transferrin test is often replaced by the total iron-binding capacity of plasma (TIBC): the transferrin test directly measures the protein in plasma, while TIBC is an indirect measure of the plasma’s capacity to bind iron. Although transferrin and TIBC are two different tests, they follow the same pattern and have essentially the same meaning, so much so that many laboratories carry out only one or the other. Some laboratories can also determine transferrin saturation, defined as the ratio between the sites occupied by iron atoms and the sites available for binding; this parameter also closely reflects changes in transferrin in response to the body’s metabolic adjustments, and it is used in much the same way as transferrin and TIBC.

How is the sample collected for testing?
The sample is taken from a vein in the arm.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
Transferrin or total iron-binding capacity (TIBC) is typically prescribed together with serum iron and ferritin in people suspected of having abnormal iron metabolism. When iron deficiency develops, serum iron falls while transferrin and TIBC rise to meet the body’s physiological needs. In fact, only about a third of circulating transferrin is saturated with iron; the rest remains available to bind the metal and forms a kind of functional reserve.
Conversely, in iron overload, such as after repeated transfusions or in haemochromatosis, serum iron rises while transferrin and TIBC may remain within the reference range or even fall. Because transferrin is a protein made by the liver, it can also be used as a marker of liver function. In addition, transferrin reflects dietary protein intake and can therefore be used to monitor nutritional status.

When is it requested?
Transferrin and total iron-binding capacity (TIBC) are not first-line tests but are prescribed when the first symptoms of iron overload or, more commonly, iron deficiency appear, whatever the underlying cause. In any case, transferrin is generally prescribed together with serum iron and ferritin to give a more complete picture of iron metabolism.

What does the test result mean?
High transferrin or total iron-binding capacity (TIBC) values usually indicate iron deficiency, while low values may suggest iron overload, as can happen after repeated transfusions or in haemochromatosis, as well as anaemia caused by infections and chronic inflammatory diseases, malnutrition, liver cirrhosis and nephrotic syndrome.

Is there anything else I should know?
Oral contraceptives can raise transferrin or total iron-binding capacity (TIBC), while decreases may be seen during treatment with chloramphenicol or ACTH.

1. Why do some laboratories only carry out TIBC and not transferrin?
2. Is transferrin the real iron carrier in plasma?

1. Why do some laboratories only carry out total iron-binding capacity (TIBC) and not transferrin?
Transferrin is a specific transport protein that moves iron from the stores, mainly in the liver, to the various tissues; total iron-binding capacity (TIBC), on the other hand, is an indirect measure of transferrin’s ability to bind iron. TIBC is used rather than transferrin because only about a third of transferrin is saturated with iron, while the rest remains unbound; TIBC should therefore measure the transferrin actually involved in transport rather than the total transferrin concentration, as transferrin testing does. However, the results of the two tests very often follow the same pattern and have essentially the same clinical meaning, so, at the doctor’s discretion, just one of the two may be enough.

2. Is transferrin the real iron carrier in plasma?
Yes. Transferrin is a beta-globulin made in the liver and is the main iron carrier in the circulation. Once it has picked up iron directly in the bloodstream, transferrin carries it to the various tissues and releases it after binding to a specific receptor on the cell membrane.