Ferritin vs. Transferrin: Key Differences
Ferritin measures your body's iron storage reserves, while transferrin saturation (TSAT) reflects the iron immediately available in circulation for red blood cell production and tissue use. These two markers serve complementary but distinct roles in assessing iron status 1.
Ferritin: The Storage Marker
Ferritin reflects total body iron stores held in the liver, spleen, bone marrow, and other tissues (both reticuloendothelial system and parenchymal iron stores) 1.
- Low ferritin (<15-30 ng/mL) is highly specific for iron deficiency in the absence of inflammation 1
- Ferritin is an acute-phase reactant, meaning it rises during inflammation, infection, or chronic disease regardless of actual iron stores 1
- In inflammatory conditions, the diagnostic threshold increases to 100 ng/mL—ferritin below this with inflammation suggests true iron deficiency 1
- Ferritin >100 ng/mL with TSAT <16% indicates anemia of chronic disease 1
Transferrin Saturation: The Availability Marker
TSAT measures the percentage of transferrin binding sites occupied by iron, calculated as (serum iron ÷ total iron-binding capacity) × 100 2.
- TSAT reflects circulating iron available for immediate use by erythroid precursors and other tissues 1
- Normal TSAT ranges from 20-50% 2
- TSAT <16-20% indicates insufficient iron for erythropoiesis, even when ferritin is normal or elevated 1
- TSAT is less affected by inflammation than ferritin, making it more reliable in inflammatory states 1
- TSAT >45-50% with elevated ferritin warrants evaluation for iron overload conditions like hemochromatosis 2
Clinical Integration: Why Both Matter
The combined interpretation of ferritin and TSAT distinguishes absolute iron deficiency from functional iron deficiency and anemia of chronic disease 1.
Diagnostic Patterns:
- Absolute iron deficiency (no inflammation): Ferritin <30 ng/mL AND TSAT <16% 1
- Functional iron deficiency: Normal/elevated ferritin (100-800 ng/mL) BUT TSAT <20%—iron stores exist but cannot be mobilized quickly enough for erythropoiesis 1
- Anemia of chronic disease: Ferritin >100 ng/mL AND TSAT <16% 1
- Mixed picture: Ferritin 30-100 ng/mL suggests combined true iron deficiency with inflammation 1
Critical Pitfall to Avoid:
Never rely on ferritin alone in patients with inflammation, chronic kidney disease, malignancy, or infection—ferritin will be falsely elevated, masking true iron deficiency 1. Always check inflammatory markers (CRP, ESR) alongside iron studies 1.
Practical Testing Considerations
- Fasting samples are preferred for TSAT because serum iron rises postprandially, affecting accuracy 2
- TSAT shows diurnal variation, so serial measurements should occur at consistent times 1
- In chronic kidney disease patients on erythropoiesis-stimulating agents, functional iron deficiency is common—ferritin may be 100-700 ng/mL while TSAT remains <20%, indicating need for iron supplementation 1
- High TSAT levels facilitate parenchymal iron deposition, making combined assessment crucial for evaluating iron overload toxicity risk 1