Calculating Transferrin Saturation
Transferrin saturation (TSAT) is calculated using the formula: TSAT (%) = (serum iron / TIBC) × 100. 1
The Basic Formula
- TSAT (%) = (serum iron / TIBC) × 100 is the standard calculation recommended by the American College of Physicians 1
- TIBC (total iron-binding capacity) measures the iron-binding capacity within serum and reflects the availability of iron-binding sites on transferrin 1
- Both serum iron and TIBC must be measured in the same units (typically μmol/L or μg/dL) for accurate calculation 1
Converting Transferrin to TIBC
- If your laboratory measures transferrin instead of TIBC directly, you can convert using: TIBC (μmol/L) = Transferrin (g/L) × 25.1 1
- This conversion is based on the theoretical iron fixation capacity of transferrin 2
- Direct immunochemical measurement of transferrin is now more advisable than chemical measurement of TIBC, as it offers better precision (CVs of 1.85-3.68% vs 6.17%) 3
Normal Reference Ranges
- Normal TSAT in adults is typically 20-50% 1
- TSAT <20% indicates a high proportion of vacant iron-binding sites and suggests iron deficiency 1
- TSAT <16% among adults is commonly used to confirm iron deficiency in healthy individuals without inflammation 1, 4
- TSAT >50% may indicate iron overload conditions 1
Important Clinical Caveats
- Diurnal variation: TSAT levels rise in the morning and fall at night, so timing of blood draw matters 1
- Recent meals: Serum iron concentration increases after each meal, affecting TSAT calculations 1
- Inflammation and infection: These conditions decrease serum iron concentration and can falsely lower TSAT 1
- Day-to-day variation: TSAT has greater within-individual variation than hemoglobin, so a single measurement may not be representative 1
- Post-IV iron: Do not evaluate iron parameters within 4 weeks of total dose iron infusion, as circulating iron interferes with the assay 1
Context-Specific Thresholds
- In chronic inflammatory conditions, use TSAT <20% as the diagnostic threshold rather than <16% 1
- In hemodialysis patients receiving erythropoiesis-stimulating agents, maintain TSAT >20% for adequate iron availability 1
- In chronic kidney disease, TIBC may be lower than in healthy individuals despite iron deficiency, making interpretation more challenging 1, 5