Soluble Transferrin Receptor (sTfR)

What is the Soluble Transferrin Receptor?

The transferrin receptor (TfR1, CD71) is a membrane protein that allows cells — especially developing red blood cells (erythroblasts) — to import iron from transferrin. When cells need more iron, TfR1 expression on the cell surface increases, and a soluble truncated fragment is shed into the bloodstream. That circulating fragment is what laboratories measure as the soluble transferrin receptor (sTfR).

Because sTfR concentration reflects total cellular iron demand, it goes up in conditions of true iron-deficient erythropoiesis (iron-deficiency anemia, hemolysis, accelerated erythropoiesis) and is essentially unaffected by inflammation. That last property is what makes it valuable on an anemia panel: ferritin, the standard iron-store marker, is an acute-phase reactant and can be falsely normal or elevated when inflammation coexists, hiding concurrent iron deficiency. sTfR is the marker that "sees through" inflammation.

Why sTfR is on a Modern Anemia Panel

The classic anemia work-up — CBC, ferritin, iron, TIBC/transferrin saturation — works well in patients without inflammation. The problem is that many patients with anemia also have inflammation (infection, autoimmune disease, malignancy, chronic kidney disease, obesity), and inflammation:

• raises hepcidin, which traps iron in macrophages and lowers serum iron and transferrin saturation,
• raises ferritin (acute-phase reactant), which can mask coexisting iron deficiency, and
• shortens red-cell survival, contributing to a normocytic, hyporegenerative anemia — the picture of anemia of chronic disease (also called anemia of inflammation).

Because sTfR is unaffected by inflammation, it stays normal in pure anemia of chronic disease and rises in iron-deficiency anemia — including in patients in whom ferritin is falsely "normal" because of inflammation. A typical modern anemia panel therefore looks like:

• Complete blood count (CBC) with red-cell indices (MCV, MCH, RDW)
• Reticulocyte count (ideally with reticulocyte hemoglobin content if available)
• Ferritin
• Serum iron and TIBC (or transferrin saturation)
• Soluble transferrin receptor (sTfR)
• CRP or hs-CRP, to gauge inflammation alongside ferritin
• Vitamin B12 and folate, if macrocytic anemia is suspected (Vitamin B12, folate)

The sTfR / log-Ferritin Index (Thomas Plot)

The single most useful number derived from sTfR is the sTfR / log-ferritin index, also known as the Thomas plot:

sTfR-F index = sTfR (mg/L) ÷ log₁₀( ferritin (µg/L) )

Commonly used cut-offs (Punnonen et al. 1997, Beguin 2008):

• sTfR-F index < 1: anemia of chronic disease (no iron deficiency).
• sTfR-F index > 2: iron-deficiency anemia (with or without coexisting inflammation).
• Values between 1 and 2 are an intermediate zone — clinical context, reticulocyte hemoglobin, and CRP help refine the interpretation.

The exact cut-off depends on the sTfR assay, so use the threshold recommended by your laboratory.

How to Interpret an sTfR Result

• High sTfR + low ferritin: classic iron-deficiency anemia.
• High sTfR + normal/high ferritin + elevated CRP: iron-deficiency anemia masked by inflammation; the sTfR-F index will be elevated.
• Normal sTfR + normal/high ferritin + elevated CRP: anemia of chronic disease without iron deficiency.
• High sTfR + normal ferritin + no inflammation: any cause of accelerated erythropoiesis — treated iron deficiency, hemolysis, recovery from blood loss, hereditary hemolytic anemia, or response to erythropoietin therapy.
• Low sTfR: uncommon; may reflect aplastic anemia or chronic kidney disease with reduced erythropoiesis.

sTfR is not a stand-alone diagnostic test. It is most powerful when paired with ferritin (via the sTfR-F index) and a marker of inflammation (CRP).