Is transferrin saturation the same as iron saturation?

Transferrin Saturation and Iron Saturation

Yes, transferrin saturation (TSAT) and iron saturation are the same measurement, both referring to the percentage of iron-binding sites on transferrin that are occupied by iron. 1

Definition and Calculation

• Transferrin saturation is calculated using the formula: TSAT (%) = (serum iron / TIBC) × 100, where TIBC is the total iron-binding capacity 1
• The transferrin molecule contains two binding sites for transporting iron from storage sites to erythroid progenitor cells; a TSAT of 50% indicates that half of these binding sites are occupied by iron 2
• TIBC is a measure of the iron-binding capacity within the serum and reflects the availability of iron-binding sites on transferrin 2, 1
• Transferrin can be converted to TIBC using the formula: TIBC (μmol/L) = Transferrin (g/L) × 25.1 1, 3

Clinical Interpretation

• Normal TSAT range in adults is typically 20-50% 1
• Low TSAT (<20%) indicates a high proportion of vacant iron-binding sites and is commonly used to identify iron deficiency 2, 1
• Among adults, a TSAT of less than 16% is often used to confirm absolute iron deficiency in healthy individuals without inflammation 2, 1
• In patients with chronic inflammatory conditions, TSAT below 20% is typically used as the diagnostic threshold 1
• High TSAT (>50%) may indicate iron overload conditions 1

Diagnostic Significance

• TSAT is an indicator of iron-deficient erythropoiesis rather than iron depletion 2
• TSAT is less sensitive to changes in iron stores than serum ferritin concentration 2, 4
• Using TSAT in addition to ferritin improves diagnostic accuracy for iron deficiency, especially in patients with chronic inflammatory diseases 4
• In iron-depleted subjects, both serum ferritin and transferrin saturation reflect iron status, but in iron-replete subjects, the control of iron absorption is more dependent on iron stores as reflected by serum ferritin 5

Factors Affecting Measurement

• Diurnal variation: TSAT levels rise in the morning and fall at night 2, 1
• Recent meals: Serum iron concentration increases after each meal, affecting TSAT 2, 1
• Inflammation and infection: Can decrease serum iron concentration and affect TSAT 2, 1
• Day-to-day variation: TSAT has greater variation within individuals than hemoglobin concentration 2, 1

Common Pitfalls in Interpretation

• Relying solely on TSAT without considering ferritin levels may lead to missed diagnoses 1
• Failing to account for inflammatory status when interpreting TSAT and ferritin can lead to inaccurate diagnoses 1, 6
• Not recognizing that functional iron deficiency can occur despite normal or elevated ferritin levels 2, 1
• Using TSAT as a single diagnostic criterion in children is not recommended, as it should always be combined with other indicators of iron status 7

Clinical Application

• A goal TSAT of ≥20% is generally desired to ensure adequate iron for erythropoiesis 1
• Laboratory evaluation following IV iron should include TSAT 4 to 8 weeks after the last infusion 1
• Iron parameters should not be evaluated within 4 weeks of a total dose iron infusion, as circulating iron interferes with the assay 1