What markers of iron deficiency exist beyond ferritin and Transferrin Saturation (TSAT)?

Alternative Markers of Iron Deficiency Beyond Ferritin and TSAT

Beyond ferritin and TSAT, the most clinically useful markers of iron deficiency include soluble transferrin receptor (sTfR), reticulocyte hemoglobin content (CHr/RET-He), hepcidin, zinc protoporphyrin (ZnPP)/heme ratio, and percentage of hypochromic red blood cells.

Primary Alternative Markers

Soluble Transferrin Receptor (sTfR)

• sTfR is particularly valuable in inflammatory conditions where ferritin becomes unreliable as an acute phase reactant 1
• sTfR levels are considerably elevated in iron deficiency anemia and reflect tissue iron demands 1
• This marker remains mostly normal in functional iron deficiency, making it especially useful to identify concomitant absolute iron deficiency in patients with inflammation 1
• sTfR has demonstrated higher sensitivity (compared to ferritin's 35-48% sensitivity) in patients with inflammatory conditions where ferritin is unreliable 1
• Important caveat: sTfR is also elevated with increased erythropoietic activity (hemolytic anemia, response to IV iron) and ineffective erythropoiesis, limiting specificity 1
• Major limitation: lack of routine availability and long turnaround time 1

Reticulocyte Hemoglobin Content (CHr/RET-He)

• This provides direct assessment of functional iron availability to erythropoietic tissue and is immediately available on certain auto-analyzers (Siemens and Sysmex) 1
• These are quick and reliable tests for detecting iron deficiency and the need for iron replacement 1
• RetHb content parameters can be measured on several modern automated hematology analyzers and provide insight into the adequacy of iron supply 1
• Limitations include lack of universal availability and abnormal results in inflammation and thalassemia 1

Hepcidin

• Hepcidin levels decrease with the onset of iron deficiency, even in the presence of inflammation 1
• In phase 3 HIF-PHI trials, hepcidin decreased significantly with iron deficiency: from median 105.6 ng/ml to 82.7 ng/ml in treatment groups 1
• In women with mild iron deficiency anemia, mild inflammation did not increase serum hepcidin levels 1
• Severely anemic children tend to have low hepcidin even in the presence of inflammation 1
• Major limitation: not universally available and lacks robust standardization 2

Percentage of Hypochromic Red Blood Cells

• Values exceeding 10% are compatible with iron deficiency in erythropoietin-treated patients (normal <2.5%) 1
• This appears to be a sensitive and reliable indicator for iron deficiency and helpful in diagnosing functional iron deficiency 1
• Requires specialized equipment (Technicon H-1, H-2, or H-3 automated cell counter), which limits availability 1

Zinc Protoporphyrin (ZnPP)/Heme Ratio

• The ZnPP/heme ratio in erythrocytes reflects iron availability and utilization in the bone marrow 1
• This ratio increases in iron deficiency and has been proposed as a useful screening test, especially in children 1
• Less widely available and offers no increase in diagnostic sensitivity or specificity over serum ferritin and TSAT 1

Additional Iron Parameters

Total Iron Binding Capacity (TIBC)

• TIBC increases with iron deficiency as the body attempts to maximize iron uptake 1
• In HIF-PHI trials, TIBC increased significantly in iron-deficient patients (from 45.0 mmol/l to 50.0 mmol/l) 1
• TIBC is used to calculate TSAT (serum iron divided by TIBC) 1

Serum Iron

• While serum iron alone has limited diagnostic value due to diurnal variation, it contributes to TSAT calculation 1
• In heart failure patients, low serum iron was independently associated with increased risk of all-cause death (standardized HR 0.87,95% CI 0.79-0.96) 3

Clinical Algorithm for Marker Selection

When ferritin and TSAT are discordant or inflammation is present:

1. First-line alternative: sTfR if available - most useful when ferritin is elevated due to inflammation but true iron deficiency is suspected 1
2. Second-line alternative: Reticulocyte hemoglobin content (CHr/RET-He) - if auto-analyzer is available, provides immediate functional assessment 1
3. Third-line: Percentage of hypochromic RBCs - particularly in patients receiving erythropoietin therapy 1

In pediatric populations:

• ZnPP/heme ratio is particularly useful for field studies and screening 1
• sTfR may not be increased in iron-deficient infants less than 1 year of age 1

Critical Pitfalls to Avoid

• Do not measure iron parameters within 4 weeks of total dose iron infusion, as circulating iron interferes with assays leading to spurious results 1
• Recognize that sTfR elevation is not specific to iron deficiency - it also increases with hemolysis and ineffective erythropoiesis 1
• Hepcidin measurement lacks standardization across laboratories, limiting clinical utility 2
• In chronic kidney disease patients receiving erythropoietin, functional iron deficiency can occur despite normal ferritin levels - alternative markers like hypochromic RBCs or sTfR are particularly valuable 1