What transferrin saturation level indicates iron deficiency?

Transferrin Saturation Levels Indicating Iron Deficiency

A transferrin saturation less than 16% is the standard threshold indicating iron deficiency in adults. 1

Understanding Transferrin Saturation

Transferrin saturation is calculated by dividing serum iron concentration by total iron binding capacity (TIBC) and multiplying by 100:

Transferrin saturation (%) = (Serum iron concentration [μg/dL] ÷ TIBC [μg/dL]) × 100

This measurement indicates the extent to which transferrin has vacant iron-binding sites, with lower values suggesting iron deficiency.

Diagnostic Thresholds by Population

General Adult Population

• <16%: Confirmed iron deficiency in adults 1, 2
• <20%: Suggestive of iron deficiency, warrants clinical consideration 1, 2

Chronic Kidney Disease Patients

• <20%: Indicative of iron deficiency when combined with ferritin <100 ng/mL 1
• <25%: May indicate functional iron deficiency even with elevated ferritin (up to 1200 ng/mL) 1

Pediatric Population

• <16%: Should be interpreted with caution in children aged 0.5-12 years and only considered indicative of iron deficiency when accompanied by anemia and low mean corpuscular volume 3

Clinical Considerations

1. Test Reliability: Transferrin saturation has a sensitivity of only 20% but a specificity of 93% for iron deficiency as defined by absence of stainable bone marrow iron 1

2. Confounding Factors:

   • Diurnal variation (higher in morning, lower at night)
   • Recent meals (serum iron increases after eating)
   • Inflammation or infection (decreases serum iron)
   • Day-to-day variation within individuals 1

3. Functional Iron Deficiency: Patients may have normal or elevated ferritin but low transferrin saturation, particularly in:

   • Chronic kidney disease
   • Inflammatory conditions
   • Patients receiving erythropoietin therapy 1, 4

4. Combined Assessment: For optimal diagnosis, transferrin saturation should be evaluated alongside:

   • Serum ferritin (<30 ng/mL typically indicates iron deficiency)
   • Complete blood count
   • Clinical symptoms 2, 5

Special Situations

1. Normal TSAT with Low Serum Iron: CKD patients with normal transferrin saturation but low serum iron (men <70 μg/dL, women <60 μg/dL) still have increased risk for anemia 4

2. High Ferritin with Low TSAT: In inflammatory states, ferritin may be elevated despite iron deficiency, making transferrin saturation a more reliable indicator 1, 6

3. Diagnostic Efficiency: Some research suggests that TIBC or transferrin concentration alone may outperform iron and saturation indices in predicting iron deficiency 7

By understanding these thresholds and considerations, clinicians can more accurately diagnose iron deficiency and initiate appropriate treatment to improve patient outcomes related to morbidity, mortality, and quality of life.