These tests measure the amount of iron that can be transported in your blood. Transferrin is a protein that carries iron in your blood. When iron stores are low, transferrin levels increase, and when there is too much iron, transferrin levels decrease. Transferrin is usually requested as part of the group of tests known as iron studies, which includes other tests relating to the body' s iron levels. Total iron binding capacity (TIBC) is an older test that is sometimes still performed by some labs. TIBC and transferrin give the same information — the amount of iron capable of being transported in the blood.

What is being tested?

Transferrin is the main protein in the blood that binds iron and transports it around the body. These tests either indirectly or directly measure the amount of transferrin in the blood available to transport iron. It is difficult to interpret iron levels in the blood without knowing how much of the binding protein transferrin is also present. Transferrin, TIBC (total iron binding capacity) and UIBC and are three different tests that measure the same thing – the amount of iron that can be transported in your blood.

The amount of iron that can be transported in the blood depends on the blood level of the protein transferrin. Transferrin attaches (binds) onto iron and carries it around the body in the bloodstream.

The TIBC test works by first measuring how much iron is in the blood sample, then seeing how much extra iron it is possible for that blood sample to attach to – which will depend on how much transferrin is in the blood. The UIBC is different way of measuring the same thing. The transferrin test is a newer test in which the actual amount of transferrin protein in the blood is measured directly.

Your local laboratory may perform one or more of these tests, depending on the facilities available there.

How is it used?

Transferrin or TiBC may be requested by your doctor if they have reasons to suspect you have abnormal levels of iron in your body – either too little (iron deficiency) or too much (iron overload).

Usually the doctor will request other related tests at the same time, such as the iron level in the blood and the level of ferritin a protein involved in iron storage.

When is it requested?

Transferrin or TIBC measurements may be requested when it appears that you have too much or too little iron in your body.

Your doctor may suspect you have too little iron if you have anaemia, especially if the red blood cells are small and pale (microcytic and hypochromic).

Your doctor may suspect you have too much iron if you have a family history of iron overload haemochromatosis or evidence of organ damage which could be due to too much iron.

What does the result mean?

In iron deficiency the transferrin and TIBC may be high. This may seem odd, but remember that the TIBC and transferrin are not measuring the amount of iron in the blood but the ability of the blood to carry iron. It seems that when the body lacks iron, it 'turns up’ the production of the iron-binding protein transferrin.

However, other things can turn the production of transferrin 'up’ or 'down’ apart from iron deficiency. For example, in people with chronic inflammatory conditions the transferrin (and TIBC) tend to be low. In women on the oral contraceptive pill, transferrin tends to be high.

For these reasons, your doctor will usually request other related tests as well as TIBC or transferrin, such as ferritin, if they suspect iron deficiency.

In iron overload, the transferrin and TIBC level are most useful when used together with the blood iron level to calculate the 'transferrin saturation’. In iron overload the transferrin saturation is high.

A summary of the changes in iron tests seen in various diseases of iron status is shown in the table below.

Is there anything else I should know?

TIBC and transferrin and the other tests commonly used to assess iron function can be difficult to interpret because they can be affected by so many things other than body iron levels. Your doctor will need to consider all these other things when interpreting the results. You must tell your doctor if you are taking iron supplements.

Common questions

• Are there other things beside iron deficiency that can cause anaemia?

Yes, there are numerous other causes of anaemia, however iron deficiency is the most common. If iron tests rule out iron deficiency then another cause must be sought. See the article on anaemia.

• What other factors affect the transferrin level?

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 cancer may cause changes in transferrin levels.

More information

RCPA Manual: Iron