No, TIBC and UIBC Are Not the Same—They Are Mathematically Related Components
Total iron-binding capacity (TIBC) represents the total amount of iron that transferrin can bind, whereas unbound iron-binding capacity (UIBC) measures only the available (unoccupied) iron-binding sites on transferrin. 1
Mathematical Relationship
- TIBC = Serum Iron + UIBC 1
- TIBC quantifies all iron-binding sites on transferrin (both occupied and vacant), while UIBC reflects only the vacant binding sites 1
- When you measure serum iron and add it to UIBC, you obtain the TIBC 1
Clinical Interpretation in Iron Deficiency
In iron deficiency states:
- TIBC is elevated because the body compensates by producing more transferrin to capture any available iron 2, 1
- UIBC is also elevated because most transferrin binding sites remain empty (unoccupied by iron) 1
- Serum iron is low because iron stores are depleted 1
- Transferrin saturation (TSAT) is low (<16–20%), calculated as (serum iron ÷ TIBC) × 100 2, 1
Diagnostic Accuracy Comparison
UIBC demonstrates superior diagnostic performance compared to TIBC or TSAT:
- UIBC has a diagnostic accuracy of 0.80–0.97 for identifying empty iron stores across different populations 1
- UIBC is more accurate than TIBC, serum iron, transferrin, or transferrin saturation when diagnosing depleted iron stores 1
- In women of childbearing age without inflammation, UIBC outperforms soluble transferrin receptor (sTFR) in diagnosing empty iron stores (area under ROC curve 0.830 vs. 0.793, p = 0.007) 3
Why TIBC Can Be Misleading
Common pitfall: TIBC measurements often overestimate the true iron-binding capacity of transferrin because iron binds nonspecifically to other plasma proteins, particularly albumin 4
- This nonspecific binding artificially inflates TIBC values 4
- Direct transferrin measurement by immunoassay avoids this problem and is now preferred in most populations 4
- However, in populations with genetic transferrin variants (TF BC and TF CD), immunochemical transferrin assays may produce errors, making TIBC measurement preferable in those specific groups 4
Practical Laboratory Considerations
When ordering iron studies:
- A complete iron panel includes serum iron, TIBC (or transferrin), TSAT, and ferritin 1
- TIBC and UIBC are not interchangeable tests—they measure different components of the same system 1
- If only UIBC is reported, you can calculate TIBC by adding the serum iron value 1
- Conversely, if only TIBC is reported, you can calculate UIBC by subtracting serum iron from TIBC 1
Clinical Bottom Line
TIBC and UIBC are distinct but complementary measurements: TIBC reflects total binding capacity (occupied + unoccupied sites), while UIBC reflects only the unoccupied sites. 1 In iron deficiency, both are elevated, but UIBC provides superior diagnostic accuracy for identifying truly empty iron stores. 1, 3