Serum Iron vs Ferritin: Different Markers of Iron Status
No, serum iron and ferritin are not the same - they measure different aspects of iron metabolism. 1, 2
Key Differences Between Serum Iron and Ferritin
Serum iron measures the amount of iron bound to transferrin in the bloodstream, representing iron that is currently in transit and available for immediate use in erythropoiesis 1
Serum ferritin reflects storage iron that is contained within liver, spleen, and bone marrow reticuloendothelial cells, serving as a reliable indicator of total body iron stores 1
Transferrin saturation (TSAT), which is calculated using serum iron, indicates the percentage of transferrin binding sites occupied by iron and reflects iron readily available for erythropoiesis 1
Clinical Significance of Ferritin
Ferritin is the most specific indicator available for assessing depleted iron stores, with approximately 1 μg/L of serum ferritin equaling about 10 mg of stored iron 2
Normal serum ferritin values differ by population:
- Adult men: 135 μg/L
- Adult women: 43 μg/L
- Children (6-24 months): approximately 30 μg/L 2
Interpretation thresholds:
- <15 μg/L: highly specific for iron deficiency (specificity 0.99)
- <30 μg/L: generally indicates low body iron stores
150 μg/L: rarely occurs with absolute iron deficiency 2
Important Considerations When Interpreting Iron Tests
Ferritin is an acute-phase reactant that increases during inflammation, infection, or tissue damage, which can mask depleted iron stores 2, 3
Serum ferritin primarily represents a consequence of cell stress and damage, as it is largely a leakage product from damaged cells 3
In inflammatory conditions, iron deficiency may be present despite ferritin levels up to 50-100 μg/L 2
Day-to-day variation in serum ferritin (14.5%) is significantly less than serum iron (28.5%), making ferritin a more reliable single measurement for assessing iron status 4
Limitations and Complementary Testing
Neither serum iron nor ferritin alone provides a complete picture of iron status - they should be interpreted together 1
In chronic kidney disease patients, the combination of low TSAT (<20%) and high ferritin (>800 ng/mL) is increasingly common due to inflammation inhibiting iron mobilization 5
Newer tests that may help assess iron status when ferritin and TSAT results are discordant include:
- Reticulocyte hemoglobin content
- Percentage of hypochromic red cells
- Soluble transferrin receptor 5
Serum ferritin iron (the iron content within ferritin protein) may provide a more direct measure of iron stores unconfounded by inflammation 6