Transferrin Saturation Calculation
Transferrin saturation (TSAT) is calculated by dividing serum iron by total iron-binding capacity (TIBC) and multiplying by 100: TSAT (%) = (serum iron / TIBC) × 100. 1, 2, 3
The Formula and Its Components
The standard equation is: TSAT (%) = (serum iron ÷ TIBC) × 100 1, 2, 3
Both serum iron and TIBC must be expressed in the same units (typically μg/dL or μmol/L) for the calculation to be valid 2, 3
When transferrin concentration is measured directly instead of TIBC, you can convert transferrin to TIBC using: TIBC (μmol/L) = Transferrin (g/L) × 25.1 2, 3
An alternative calculation using transferrin directly is the transferrin index: TI = serum iron / transferrin, which some laboratories prefer because transferrin assays have better precision (CV 1.85-3.68%) compared to TIBC assays (CV 6.17%) 4
Clinical Interpretation of Results
TSAT <20% indicates iron-deficient erythropoiesis and has 93% specificity for diagnosing absolute or functional iron deficiency 1, 2
TSAT <16% in adults without inflammation confirms absolute iron deficiency 2, 3
TSAT >50% suggests iron overload and warrants evaluation for hemochromatosis or other iron-loading conditions 2, 3
Important Measurement Considerations
Do not measure iron parameters within 4 weeks of intravenous iron administration because circulating iron interferes with assays and produces falsely elevated results 1, 2
The optimal timing for reassessment after IV iron is 4-8 weeks after the last infusion 1, 2
TSAT exhibits significant diurnal variation (rises in morning, falls at night) and increases after meals, so fasting morning samples provide the most reliable results 1, 2, 3
Day-to-day variation in TSAT is greater than for hemoglobin, so a single abnormal value should be confirmed with repeat testing 1, 2, 3
Why TSAT Matters More Than Ferritin in Inflammatory States
TSAT is less affected by inflammation than ferritin, making it more reliable for diagnosing iron deficiency in patients with chronic kidney disease, heart failure, inflammatory bowel disease, or cancer 1, 2
Ferritin <100 ng/mL has only 35-48% sensitivity for detecting iron deficiency, whereas TSAT <20% provides markedly higher sensitivity 2
In chronic inflammatory conditions, ferritin can be elevated (100-300 ng/mL) due to acute-phase reactivity while TSAT remains low, revealing functional iron deficiency where iron is sequestered and unavailable for erythropoiesis 1, 2
Common Pitfalls to Avoid
Never rely on serum iron alone because it has high intra-individual variability and does not reflect total body iron stores 2
Do not interpret ferritin without TSAT in patients with known or suspected inflammation, as ferritin alone will miss functional iron deficiency 1, 2
In chronic kidney disease patients, TIBC may be lower than in healthy individuals despite iron deficiency due to malnutrition and inflammation, so TSAT remains more diagnostically useful than TIBC alone 5, 6, 7
Patients with normal TSAT but low serum iron (below 70 μg/dL in men, 60 μg/dL in women) may still be at risk for anemia in CKD stages 1-4, so consider both parameters together 5