Understanding the Difference Between TIBC and UIBC
Total Iron Binding Capacity (TIBC) represents the maximum amount of iron that transferrin can bind in the serum, while Unsaturated Iron Binding Capacity (UIBC) measures only the remaining available iron-binding sites on transferrin that are not currently occupied by iron.
Mathematical Relationship
- TIBC equals the sum of serum iron plus UIBC 1, 2
- The formula is: TIBC = Serum Iron + UIBC 2, 3
- Alternatively expressed: UIBC = TIBC - Serum Iron 3, 4
- Transferrin saturation (TSAT) is calculated as: (Serum Iron / TIBC) × 100 2, 5
Clinical Significance of Each Measurement
TIBC (Total Iron Binding Capacity)
- TIBC measures the iron-binding capacity within the serum and reflects the total availability of iron-binding sites on transferrin 2, 5
- TIBC increases when serum iron concentration and stored iron are low, as the body produces more transferrin to capture available iron 2
- In some laboratories, TIBC is calculated from the sum of serum iron and UIBC, while in others it is calculated indirectly from the transferrin concentration in serum 1
- TIBC can be converted from transferrin using the formula: TIBC (μmol/L) = Transferrin (g/L) × 25.1 5
UIBC (Unsaturated Iron Binding Capacity)
- UIBC indicates the extent to which transferrin has vacant iron-binding sites 2
- A high UIBC indicates a high proportion of vacant iron-binding sites, suggesting iron deficiency 2
- UIBC demonstrates better diagnostic accuracy than serum iron, transferrin, or transferrin saturation when identifying empty iron stores, with diagnostic accuracy of 0.80-0.97 across different populations 2
- UIBC is more accurate than soluble transferrin receptor (sTFR) in diagnosing empty iron stores in women without inflammation 6
Diagnostic Applications
Iron Deficiency
- Elevated UIBC is a more accurate diagnostic marker for empty iron stores than traditional measures like serum iron, transferrin, or transferrin saturation 2
- In iron deficiency, TIBC increases as the body produces more transferrin, resulting in elevated UIBC 2
- A TSAT of less than 16% (calculated from serum iron and TIBC) among adults is often used to confirm iron deficiency 2, 5
Iron Overload
- In iron overload conditions, UIBC may be decreased or near zero as transferrin becomes saturated with iron 1
- Standard colorimetric methods for measuring TIBC may give falsely high results in iron-saturated plasma due to nontransferrin iron 7, 4
- Transferrin is normally not more than 50% saturated with iron in healthy states, ensuring no free iron is available for microbial growth 5
Laboratory Measurement Considerations
- The indirect iron saturation excess method for UIBC determination is preferred over direct adsorption methods for TIBC, which give falsely high results due to unspecific binding of saturating iron to serum proteins 4
- Standard colorimetric methods for measuring TIBC are accurate except in iron-saturated plasma, where nontransferrin iron can increase results 7
- UIBC measurements in physiological medium (pH 7.4) require standard addition method due to strong protein influence on analytical signals 3
- Serum iron concentration increases after each meal and shows diurnal variation, which affects calculated TIBC values 5
Common Pitfalls
- In chronic kidney disease patients, TIBC may be lower than in healthy individuals despite iron deficiency, making interpretation more challenging 5
- Inflammation and infection can affect serum iron concentration, thereby impacting both TIBC and UIBC calculations 1, 5
- Day-to-day variation is greater for TSAT (which depends on TIBC) than for hemoglobin concentration 5
- Plasma samples are unsuitable for direct TIBC assays and may yield inaccurate results 8