# Soluble Transferrin Receptor (sTfR)

> Soluble transferrin receptor (sTfR) reference ranges and clinical use on the anemia panel. Distinguishes iron-deficiency anemia from anemia of chronic disease via the sTfR / log-ferritin (Thomas) index.

*Source: [https://www.health3.app/biomarkers/stfr](https://www.health3.app/biomarkers/stfr)*

A modern anemia-panel marker that tells iron-deficiency anemia apart from anemia of chronic disease.

### On This Page

- What sTfR measures
- Measurement units
- Reference ranges
- Why sTfR is on the anemia panel
- sTfR / log-ferritin index (Thomas plot)
- How to interpret your result
- Related biomarkers & tools
- Scientific references

## 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.

## Measurement Units

sTfR is most commonly reported in **mg/L** in clinical laboratories. Some assays report it in nmol/L or as a relative ratio. Because reference ranges are assay-specific and not yet harmonized across manufacturers, sTfR is best interpreted against the reference range printed on the individual lab report rather than a generic table.

## Typical Reference Ranges

Reference ranges are assay-dependent. The values below are commonly cited adult ranges; your laboratory's reference values take precedence.

| Population | Unit | Typical reference range | Source |
| --- | --- | --- | --- |
| Adults (general) | mg/L | 2.2 – 5.0 | Beguin, 2008 |
| Adults (general) | nmol/L | ~25 – 60 | Beguin, 2008 |

## 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](https://www.health3.app/biomarkers/ferritin.html)
- [Serum iron](https://www.health3.app/biomarkers/iron.html) 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](https://www.health3.app/biomarkers/vitaminb12.html), [folate](https://www.health3.app/biomarkers/vitaminb9_plasma.html))

## 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 the relevant laboratory.

## Understanding 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).

## Related Biomarkers & Tools

- [**Ferritin**](https://www.health3.app/biomarkers/ferritin.html)

 The standard iron-store marker. Read alongside sTfR — ferritin is an acute-phase reactant and can be falsely "normal" in inflammation.
- [**Iron (serum)**](https://www.health3.app/biomarkers/iron.html)

 Reflects circulating iron bound to transferrin; combined with TIBC gives transferrin saturation.
- [**Iron Saturation Calculator**](https://www.health3.app/tools/iron-saturation-calculator)

 Computes transferrin saturation from serum iron and TIBC. It is used alongside sTfR and ferritin for the full iron picture.
- [**Blood Test Unit Converter**](https://www.health3.app/tools/blood-test-unit-converter)

 Convert sTfR and other biomarkers between common laboratory units.

## Academic References

1. Skikne BS, Flowers CH, Cook JD. Serum transferrin receptor: a quantitative measure of tissue iron deficiency (1990). *Blood*. [DOI: 10.1182/blood.V75.9.1870.1870](https://doi.org/10.1182/blood.V75.9.1870.1870)
2. Punnonen K, Irjala K, Rajamäki A. Serum transferrin receptor and its ratio to serum ferritin in the diagnosis of iron deficiency (1997). *Blood*. [DOI: 10.1182/blood.V89.3.1052](https://doi.org/10.1182/blood.V89.3.1052)
3. Weiss G, Goodnough LT. Anemia of chronic disease (2005). *New England Journal of Medicine*. [DOI: 10.1056/NEJMra041809](https://doi.org/10.1056/NEJMra041809)
4. Beguin Y. Soluble transferrin receptor for the evaluation of erythropoiesis and iron status (2008). *Haematologica / Clinica Chimica Acta*. [DOI: 10.1016/S0009-8981(03)00101-0](https://doi.org/10.1016/S0009-8981(03)00101-0)

### ⚠️ Important Medical Information

This reference page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

sTfR reference ranges and sTfR/log-ferritin index thresholds are assay-dependent and not yet fully harmonized between manufacturers. Always review your lab results with a qualified healthcare provider using the reference range printed on your report.