Calculation and Interpretation of TIBC and Percent Saturation in Iron Deficiency Anemia
Total Iron Binding Capacity (TIBC) is calculated from laboratory measurements of serum iron and transferrin, while percent saturation is calculated as (serum iron ÷ TIBC) × 100, with values below 20% strongly indicating iron deficiency requiring treatment.
Calculation of TIBC and Percent Saturation
TIBC Calculation
- TIBC is a measure of the iron-binding capacity within serum and reflects the availability of iron-binding sites on transferrin 1
- It can be measured directly in the laboratory or calculated from transferrin measurements
- TIBC increases when serum iron concentration and stored iron are low 1
- Normal TIBC range is typically 250-450 μg/dL (45-80 μmol/L)
Percent Saturation Calculation
- Transferrin saturation (%) = (Serum iron concentration [μg/dL] ÷ TIBC [μg/dL]) × 100 1
- This calculation indicates the extent to which transferrin has vacant iron-binding sites 1
- A low transferrin saturation indicates a high proportion of vacant iron-binding sites, suggesting iron deficiency 1
Interpretation in Iron Deficiency Anemia
Diagnostic Thresholds
- Transferrin saturation <20% is considered indicative of iron deficiency 1, 2
- Transferrin saturation <16% is often used to confirm iron deficiency in adults 1
- In chronic kidney disease patients, a TSAT <20% is considered indicative of iron deficiency 1
Patterns in Iron Deficiency
- In iron deficiency:
Clinical Significance
- Transferrin saturation is more sensitive than either serum iron or TIBC alone for diagnosing iron deficiency 2, 4
- A transferrin saturation <20% has high sensitivity for diagnosing both absolute and functional iron deficiency 2
- TIBC >70 μmol/L combined with percent saturation <16% has a 93% predictive value for iron deficiency 5
Factors Affecting Test Results
Factors Affecting Serum Iron
- Diurnal variation (higher in morning, lower at night) 1
- Recent meals (increases after eating) 1
- Infections and inflammation (decrease levels) 1
- Day-to-day variation within individuals 1
Factors Affecting TIBC
- Inflammation, chronic infection, malignancies, liver disease, nephrotic syndrome, and malnutrition can lower TIBC readings 1
- Oral contraceptive use and pregnancy can raise TIBC readings 1
- TIBC has less day-to-day variation than serum iron concentration 1
- TIBC is less sensitive to iron deficiency than serum ferritin, as changes occur after iron stores are depleted 1
Factors Affecting Percent Saturation
- All factors affecting serum iron and TIBC
- Diurnal variation (larger than for hemoglobin or hematocrit) 1
- Day-to-day variation within persons 1
Clinical Application
When to Test
- In patients with anemia, especially microcytic anemia
- In patients with symptoms of iron deficiency (fatigue, pica, restless legs syndrome) 3
- In high-risk populations (menstruating women, pregnant women, patients with chronic kidney disease) 3
Interpretation Pitfalls
- Transferrin saturation is an indicator of iron-deficient erythropoiesis rather than iron depletion 1
- It is less sensitive to changes in iron stores than serum ferritin 1
- In inflammatory states, ferritin may be falsely elevated as an acute phase reactant, making transferrin saturation more reliable 2
- In chronic kidney disease, interpretation must consider blood losses from hemodialysis and effects of erythropoietin therapy 1, 2
Treatment Thresholds
- Consider iron supplementation when:
Monitoring Treatment Response
- Monitor hemoglobin, ferritin, and transferrin saturation every 3 months during active treatment 2
- Target increase in hemoglobin of at least 2 g/dL within 4 weeks of treatment 2
- Iron parameters should not be checked within 4 weeks of IV iron administration as circulating iron interferes with the assay 2
By understanding the calculation and interpretation of TIBC and percent saturation, clinicians can more accurately diagnose and manage iron deficiency anemia, leading to improved patient outcomes.