How to Calculate Transferrin Saturation
Transferrin saturation is calculated by dividing serum iron concentration (µg/dL) by total iron-binding capacity (TIBC) (µg/dL) and multiplying by 100 to express as a percentage. 1
The Formula
Transferrin Saturation (%) = [Serum Iron (µg/dL) / TIBC (µg/dL)] × 100
For your specific patient with restless leg syndrome:
- Serum iron = 55 µg/dL
- TIBC = 300 µg/dL
- Transferrin saturation = (55 / 300) × 100 = 18.3%
Clinical Interpretation for Restless Leg Syndrome
This patient's transferrin saturation of 18.3% is below the 20% threshold and indicates iron deficiency requiring treatment. 2
According to the 2025 American Academy of Sleep Medicine guidelines for RLS management, iron supplementation should be instituted when transferrin saturation is < 20% 2. Your patient meets this criterion.
Treatment Implications Based on Iron Parameters
For patients with RLS and transferrin saturation < 20%:
- If serum ferritin ≤ 75 ng/mL: Use oral OR intravenous iron 2
- If serum ferritin 75-100 ng/mL: Use ONLY intravenous iron 2
- If serum ferritin > 100 ng/mL: Recent evidence suggests IV iron may still be beneficial up to ferritin levels of 300 µg/L 3
Important Clinical Considerations
Timing of Blood Draw
Iron studies should ideally be obtained in the morning after avoiding all iron-containing supplements and foods for at least 24 hours prior to blood draw 2. This is critical because serum iron concentration increases after each meal and exhibits diurnal variation (rising in morning, falling at night) 1.
Factors Affecting Accuracy
Several conditions can alter transferrin saturation independent of true iron status 1:
Factors that LOWER TIBC (falsely elevating transferrin saturation):
- Inflammation or chronic infection
- Malignancies
- Liver disease
- Nephrotic syndrome
- Malnutrition
Factors that RAISE TIBC (falsely lowering transferrin saturation):
- Oral contraceptive use
- Pregnancy
Common Pitfall: Inflammation
Because serum iron decreases with inflammation while TIBC may also decrease, transferrin saturation can be misleading in inflammatory states 1. The day-to-day variation is larger for transferrin saturation than for hemoglobin or hematocrit 1.
Assay Variability
Be aware that different laboratory methods may yield slightly different results. Some laboratories measure transferrin directly and convert it to TIBC, while others measure TIBC directly 4. When transferrin is measured, an alternative calculation is the transferrin index (TI = Fe/TRF), which may have better precision 4.
Additional Testing Needed
You must also obtain serum ferritin to complete the iron status assessment and determine the appropriate route of iron supplementation (oral vs. intravenous) 2. Transferrin saturation alone is insufficient for treatment decisions in RLS.
The combination of transferrin saturation < 20% with low ferritin has 93% specificity for iron deficiency in adults 1.