Soluble transferrin receptor (sTfR) test explained

Transferrin receptors are proteins on the surface of cells, through which cells take up iron-loaded transferrin from the blood. When a cell — especially a developing red blood cell in the bone marrow — is short of iron, it produces more of these receptors so that it can draw iron from the blood more efficiently. Some of the receptors shed from the cell surface and enter the blood in a soluble form — these are the soluble transferrin receptors (sTfR). The greater the iron demand of the tissues, therefore, the more sTfR is found in the blood. Iron status is most often assessed using several tests together — iron in the blood, transferrin, total iron-binding capacity (TIBC) and ferritin (according to MedlinePlus). sTfR is regarded as an additional marker, used when the results of these tests conflict, or when inflammation is present at the same time that could raise ferritin despite iron deficiency.

• When the results of a complete blood count and of the other iron markers (iron, ferritin, TIBC) are not clear-cut
• When possible iron deficiency occurs together with inflammation or a chronic illness that can make ferritin misleadingly normal or raised
• When iron deficiency anaemia is suspected and additional confirmation is needed before treatment is started
• To monitor iron stores during pregnancy or in people who do sport, when the other markers fluctuate
• When there is long-standing tiredness, weakness or other symptoms related to iron metabolism, and the first-line tests do not give a clear answer

The sources cited on this page (MedlinePlus, NHS) give no specific numeric reference interval for the sTfR test — the limit depends on the method the laboratory uses, so you will find the exact interval on your own laboratory report sheet. According to MedlinePlus, the iron markers (iron, transferrin, TIBC, ferritin) help to assess iron deficiency or iron overload. sTfR is most often used as an additional marker when the other results are unclear.

A raised sTfR

A raised sTfR level indicates an increased iron demand in the cells — most often because of iron deficiency, or because red blood cell production has become more active. Iron deficiency can arise from too little iron in the diet, from blood loss, or from an increased iron requirement in the body. Unlike ferritin, sTfR is less affected by inflammation, which is why it helps to tell true iron deficiency apart from a change in ferritin caused by inflammation.

Causes usually considered

• Iron deficiency anaemia — too little iron in the diet, or poor absorption of it (MedlinePlus)
• Blood loss — heavy periods, bleeding in the digestive tract due to ulcers, inflammatory bowel disease or haemorrhoids (NHS)
• An increased need for iron — pregnancy or rapid growth (MedlinePlus)
• More active red blood cell production in the bone marrow for other reasons

What to discuss with a doctor

• A complete blood count and the other iron markers (iron, ferritin, TIBC) — assessed together for a more accurate picture
• A search for a possible source of blood loss, especially if there is no obvious cause (for example an examination of the digestive tract)
• Diet, and conditions that interfere with iron absorption
• When to repeat the test, so that the change over time can be seen

Related tests

• Ferritin — iron stores
• Iron in the blood
• Transferrin and transferrin saturation
• TIBC — total iron-binding capacity
• Haemoglobin