Absolute Iron Deficiency Anemia (IDA) Definition
Absolute iron deficiency anemia is defined as low hemoglobin concentration combined with depleted or absent total body iron stores, confirmed by serum ferritin <30 ng/mL and/or transferrin saturation <20%. 1, 2
Core Diagnostic Components
Anemia Threshold
- Hemoglobin below the lower limit of normal for the relevant population and laboratory performing the test 1
- WHO criteria: Hb <130 g/L in men over 15 years, <120 g/L in non-pregnant women over 15 years, <110 g/L in pregnant women (2nd/3rd trimester) 1, 3
Iron Deficiency Confirmation
- Serum ferritin is the single most useful marker for confirming absolute iron deficiency 1
- Ferritin <30 ng/mL indicates low body iron stores and defines absolute iron deficiency 1, 3
- Transferrin saturation <20% supports the diagnosis when ferritin is equivocal 1, 3
Distinguishing Absolute from Functional Iron Deficiency
This distinction is critical in clinical practice, particularly in cancer and chronic inflammatory conditions:
- Absolute iron deficiency (AIDA): TSAT <20% AND ferritin <30 ng/mL, representing truly depleted iron stores 1
- Functional iron deficiency (FIDA): TSAT 20-50% AND ferritin 30-800 ng/mL, where adequate stores exist but cannot be mobilized due to inflammation 1
Clinical Confirmation
- A hemoglobin rise ≥10 g/L within 2 weeks of iron therapy is highly suggestive of absolute iron deficiency, even when iron studies are equivocal 1, 3
- This therapeutic response provides functional confirmation when laboratory values are borderline 1
Additional Laboratory Features
- Microcytosis (low MCV) and hypochromia (low MCH) typically accompany absolute iron deficiency 1, 3
- Mean corpuscular hemoglobin (MCH) may be more sensitive than MCV for detecting iron deficiency 1
- These red cell indices lose sensitivity in the presence of chronic disease, thalassemia, or vitamin B12/folate deficiency 1
Important Caveats
In inflammatory conditions, ferritin <100 μg/L may still indicate iron deficiency despite the higher threshold, as ferritin is an acute phase reactant 3. The British Society of Gastroenterology emphasizes that transferrin saturation becomes particularly helpful when false-normal ferritin is suspected in these contexts 1.