Soluble Transferrin Receptor (sTfR) Test
Soluble transferrin receptor (sTfR) is elevated in iron deficiency and remains unaffected by inflammation, making it the superior test for distinguishing iron-deficiency anemia from anemia of chronic disease when ferritin is unreliable due to inflammatory conditions. 1
What sTfR Indicates
sTfR reflects two key physiological states:
- Tissue iron deficiency - When cells lack iron, they upregulate transferrin receptors on their surface, leading to increased sTfR in serum
- Erythropoietic activity - Increased red blood cell production also raises sTfR levels
The critical advantage: sTfR is NOT an acute phase reactant, unlike ferritin which falsely elevates during inflammation, infection, or chronic disease 2, 1.
Clinical Application: Distinguishing IDA from Anemia of Chronic Disease
The Diagnostic Challenge
When evaluating anemia with inflammation present, ferritin becomes unreliable because it rises as an acute phase reactant. This creates a diagnostic dilemma where:
- TSAT may be <20% in both conditions
- Ferritin may be 100-700 ng/mL (or higher) in both conditions 3
How to Use sTfR
In iron deficiency anemia:
- sTfR is elevated (typically >2.5-3.0 mg/L depending on assay) 2, 4
- Remains high even when inflammation is present
In pure anemia of chronic disease:
In combined conditions (ACD + IDA):
- sTfR is elevated, indicating coexistent true iron deficiency 5
- This is the scenario where sTfR provides maximum clinical value
The sTfR/log Ferritin Index: Superior Discrimination
The sTfR/log ferritin index outperforms either test alone, particularly in chronic disease 2. This ratio:
- Provides superior discrimination with AUC of 0.87-0.994 6, 7
- Is especially valuable in the "ferritin grey zone" (10-100 ng/mL) where diagnosis is most difficult 7
- More than doubles IDA detection from 41% (ferritin alone) to 92% (when using ferritin, sTfR, and sTfR index together) 6
Practical Diagnostic Algorithm
When ferritin is <45 ng/mL without inflammation:
- Iron deficiency is present; sTfR not needed 2
When ferritin is 45-150 ng/mL with inflammation or chronic disease:
- Order sTfR
- If sTfR elevated → Iron deficiency present (treat with iron)
- If sTfR normal → Pure anemia of chronic disease (treat underlying condition)
- Calculate sTfR/log ferritin index for enhanced accuracy 2, 1
When ferritin is >150 ng/mL:
- Iron deficiency unlikely even with inflammation 2
- If TSAT <20%, consider functional iron deficiency or inflammatory block 3, 1
Important Caveats and Limitations
When sTfR Can Be Falsely Elevated (Not Iron Deficiency):
- Hemolytic anemias - increased erythropoietic drive 2
- Thalassemias - ineffective erythropoiesis 2
- Recent response to IV iron - stimulated erythropoiesis 1
- Hemoglobin E disease 2
Practical Limitations:
- Not routinely available in most hospitals 2, 1
- Long turnaround time when sent to reference labs 1
- Different assay methods have different cutoff values - chemiluminescent (2.91 mg/L), immunoturbidimetric (6.70 mg/L), immunonephelometric (2.48 mg/L) 4
- Not useful in infants <1 year - may remain normal despite iron deficiency 8
Performance Characteristics
Based on meta-analysis of 18 studies 9:
- Sensitivity: 86% for detecting iron deficiency
- Specificity: 75%
- Positive likelihood ratio: 3.85
- Negative likelihood ratio: 0.19
- AUC: 0.912
The sTfR index performs even better with AUC of 0.87-0.994 in various studies 6, 7.
Alternative Tests When sTfR Unavailable
If sTfR is not available, consider:
- Reticulocyte hemoglobin content (CHr or RET-He) - directly assesses functional iron availability, immediately available on certain analyzers (Siemens, Sysmex) 1
- Percent hypochromic red cells - >10% indicates iron deficiency, requires specialized equipment 3
- Empiric trial of IV iron - give 50-125 mg weekly for 8-10 doses; if no erythropoietic response, inflammatory block is present rather than true iron deficiency 3