What is the transferrin saturation for a patient with a serum iron concentration of 64 µg/dL and a total iron‑binding capacity of 310 µg/dL?

Transferrin Saturation Calculation

The transferrin saturation is 20.6%, calculated as (64 ÷ 310) × 100. 1

Calculation Method

• Transferrin saturation (TSAT) is calculated using the formula: TSAT (%) = (serum iron / TIBC) × 100, as recommended by the American College of Physicians. 1

• For this patient:

  • Serum iron = 64 µg/dL
  • TIBC = 310 µg/dL
  • TSAT = (64 ÷ 310) × 100 = 20.6%

Clinical Interpretation of This Result

This TSAT of 20.6% falls at the lower threshold of normal and warrants careful clinical correlation with ferritin and inflammatory markers. 1

Normal Range Context

• The normal TSAT range in adults is typically 20–50%, as indicated by the American College of Physicians. 1

• A TSAT of 20.6% is technically within normal range but sits at the very lower boundary, meaning iron availability for erythropoiesis may be marginal. 1

Diagnostic Thresholds for Iron Deficiency

• In adults without inflammation, a TSAT < 16% confirms absolute iron deficiency, according to the CDC. 1

• In patients with chronic inflammatory conditions (heart failure, chronic kidney disease, inflammatory bowel disease, cancer), TSAT < 20% is the diagnostic threshold for iron deficiency. 1

• This patient's TSAT of 20.6% is just above the inflammatory-state cutoff, meaning iron deficiency cannot be ruled out without additional testing. 1

Essential Next Steps

You must measure serum ferritin and C-reactive protein to properly interpret this borderline TSAT result. 1

Required Additional Testing

• Serum ferritin is essential because:

  • Ferritin < 30 ng/mL (without inflammation) confirms iron deficiency. 1
  • Ferritin < 100 ng/mL (with inflammation) confirms iron deficiency. 1
  • Ferritin 100–300 ng/mL with TSAT < 20% defines functional iron deficiency in inflammatory states. 1

• C-reactive protein must be measured because:

  • Inflammation falsely elevates ferritin, masking true iron deficiency. 1, 2
  • The TSAT threshold for diagnosing iron deficiency shifts from < 16% to < 20% when inflammation is present. 1

Complete Blood Count

• Obtain hemoglobin, hematocrit, MCV, and reticulocyte count to assess for anemia and determine its severity. 1, 2

• Microcytic anemia (low MCV) with this borderline TSAT strongly suggests iron deficiency. 2

Common Diagnostic Pitfalls

• Do not rely on TSAT alone to exclude iron deficiency—a TSAT of 20.6% may still represent inadequate iron availability in the setting of inflammation or functional iron deficiency. 1

• Serum iron and TIBC have high day-to-day variability and are affected by recent meals, diurnal changes, and inflammation; ferritin and TSAT together provide far more reliable diagnostic information. 1, 2

• Normal serum iron does not exclude iron deficiency because of poor diagnostic accuracy and high variability. 2

• In chronic inflammatory conditions, ferritin up to 100–300 ng/mL may still indicate true iron deficiency despite appearing "normal," because ferritin is an acute-phase reactant. 1

Clinical Decision Algorithm

1. If CRP is normal (no inflammation):

   • Ferritin < 30 ng/mL → Absolute iron deficiency confirmed. 1
   • Ferritin ≥ 30 ng/mL → Iron deficiency unlikely; investigate other causes of anemia. 1

2. If CRP is elevated (inflammation present):

   • Ferritin < 100 ng/mL → Absolute iron deficiency confirmed. 1
   • Ferritin 100–300 ng/mL with TSAT 20.6% → Functional iron deficiency; intravenous iron is required because hepcidin blocks intestinal absorption. 1
   • Ferritin > 300 ng/mL → Iron deficiency unlikely; consider anemia of chronic disease. 1