Soluble Transferrin Receptor
When iron deficiency anemia is suspected
A blood sample drawn from a vein in your arm
Soluble transferrin receptors are proteins found in blood that can be elevated with iron deficiency. This test measures the amount of soluble transferrin receptors to help detect and evaluate iron deficiency and aid in the diagnosis of iron deficiency anemia.
Iron is an essential nutrient absorbed from the diet. Transferrin is the main protein in the blood that binds to iron and carries it to tissues and cells throughout the body. About 80% of iron is transported to the bone marrow, where it is incorporated into the hemoglobin in red blood cells (RBCs), and the rest is stored in other cells and tissues as ferritin and hemosiderin.
When transferrin binds to iron, they form a complex. These complexes bind to transferrin receptors found on the surface of cells and the iron is then transported into the cells. Transferrin receptors are cleaved from the surface of cells, enter the bloodstream, and become soluble transferrin receptors (sTfR).
The number of transferrin receptors found on the surface of cells correlates with the level of iron within cells. When the iron level drops, the cells produce more transferrin receptors. As more receptors are produced, more are cleaved from cell surfaces and enter the blood, increasing the level of soluble transferrin receptors. Thus, measuring the level of sTfR is one way of evaluating the amount of iron available in the body.
Red blood cells have a lifespan of about 120 days. Because of this, the bone marrow must continually produce new RBCs. If there is insufficient iron taken in through the diet and/or increased iron loss (e.g., bleeding), iron reserve stored in the body is decreased. Over time, this can lead to iron deficiency, as the amount of iron stored in the body exhausts and the demand for iron exceeds the supply. As a result, fewer RBCs are produced and those newly formed RBCs are also smaller (microcytic) and contain less hemoglobin (hypochromic). This is a condition called iron deficiency anemia.
Usually, a ferritin test is used to evaluate the amount of iron reserve in the body and to help diagnose iron deficiency anemia. However, the ferritin level can be elevated when an individual has inflammation or a chronic disease, such as autoimmune disorders or some cancers. In these conditions, the ferritin test is not useful and a test for soluble transferrin receptors may be ordered.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
The soluble transferrin receptor (sTfR) test is primarily ordered to help distinguish between anemia that is caused by iron deficiency and anemia that is caused by inflammation or a chronic illness (so-called anemia of chronic disease or inflammation). It is not a routine test but may be ordered if other tests that evaluate the amount of iron in the body do not provide conclusive information.
Typically, a group of iron tests is ordered to evaluate iron availability if a person has anemia and/or if iron deficiency is suspected. These tests may include iron, transferrin, TIBC (total iron-binding capacity, which is either measured or calculated based on the level of transferrin), transferrin saturation (calculated based on levels of iron and TIBC), and ferritin.
Ferritin is usually the preferred test to evaluate stored iron, but it is an acute phase reactant, which means that it may be increased with inflammation or with chronic diseases such as autoimmune disorders, some cancers, and chronic infections. In these cases, ferritin may not be a good measure of stored iron. Since sTfR is not an acute phase reactant, it may be ordered as an alternative to ferritin if a chronic illness is present or suspected.
When is it ordered?
- Weakness, fatigue
- Lack of energy
This test may also be ordered when a complete blood count (CBC) shows decreased hemoglobin and small red blood cells (low MCV).
It may be ordered when a person has a normal or near normal ferritin level, but iron deficiency is still suspected. It may be ordered as an alternative to ferritin when a person has a chronic illness and/or inflammation is present or suspected.
What does the test result mean?
The level of soluble transferrin receptor (sTfR) will be increased with iron deficiency. If the sTfR level is elevated in someone with anemia, then it is likely that the person has iron deficiency anemia. If the level is within the normal range, then it is more likely that the person's anemia is due to another cause, such as chronic disease or inflammation.
A decreasing sTfR level indicates an increase in iron reserve in a person being treated with iron supplements. A low level of sTfR may be seen with hemochromatosis (a condition associated with iron overload), but the test is not usually used to monitor this condition.
Is there anything else I should know?
Those of African American descent may have moderately increased sTfR levels, as will those who live at higher elevations.
Testing may not be useful in those who have inherited disorders that affect hemoglobin production or function, such as thalassemia or sickle cell anemia. Test results may be elevated in those with red blood cell breakage (hemolysis) or recent blood loss and will not be accurate in those who have had a recent blood transfusion.
An elevated sTfR level is not as specific for iron deficiency as is, for example, a very low level of ferritin.
When a person is not iron deficient, the amount of sTfR present in the blood reflects the rate of red blood cell production by the bone marrow. Soluble transferrin receptor testing is not routinely performed to evaluate this process.
Why not just order an iron test?
A single iron test does not provide the full picture of the amount of iron that a person has stored and available. It is usually necessary to evaluate the results of several related iron tests together. Ferritin is usually the preferred test to evaluate stored iron, but it is an acute phase reactant, meaning that it may be increased with inflammation or chronic diseases. In these cases, ferritin may not be a good measure of stored iron. The soluble transferrin receptor test is not a routinely order test, but since it is not an acute phase reactant, it may be ordered as an alternative to ferritin to evaluate stored iron when a chronic illness is present or suspected.
Can I have iron deficiency if I feel fine?
Yes. Early deficiency usually will not cause any symptoms and when symptoms do emerge, they may be mild or nonspecific enough – such as fatigue – that they are attributed to something else.
Can my test for sTfR be performed in my doctor's office?
Why does my doctor want to send a second sTfR test to the same laboratory as the first test?
Test methods and reference (normal) ranges for sTfR will vary slightly from laboratory to laboratory. Your healthcare provider may prefer to send your samples to the same laboratory to make results more directly comparable.
Should I just take iron supplements?
On This Site
Elsewhere On The Web
American Society of Hematology: Iron-deficiency Anemia
Centers for Disease Control and Prevention: FastStats - Anemia or Iron Deficiency
MedlinePlus Medical Encyclopedia: Iron deficiency anemia
National Heart Lung and Blood Institute: What is Iron-Deficiency Anemia?
National Institutes of Health Office of Dietary Supplements: Iron
National Hematologic Diseases Information Service: Anemia of Inflammation and Chronic Disease
Iron Disorders Institute: About Iron