How to differentiate between microcytic hypochromic anemia causes, including sideroblastic anemia, anemia of chronic disease, iron deficiency anemia, and thalassemia, based on serum iron, ferritin, Total Iron Saturation (TSAT) and Total Iron Binding Capacity (TIBC) levels?

Differentiating Microcytic Hypochromic Anemia by Iron Studies

Use serum ferritin as your primary discriminator: <30 μg/L indicates iron deficiency anemia, while ferritin >100 μg/L with low transferrin saturation (<20%) indicates anemia of chronic disease; normal ferritin with normal/high TSAT suggests thalassemia; and elevated ferritin with high TSAT points to sideroblastic anemia. 1

Primary Diagnostic Algorithm

Step 1: Measure Serum Ferritin First

Ferritin is the single most powerful test for differentiating causes of microcytic anemia 1, 2:

• <15 μg/L: Diagnostic of absent iron stores (iron deficiency anemia) 1, 2
• <30 μg/L: Indicates low body iron stores (iron deficiency anemia) 1
• <45 μg/L: Optimal clinical cut-off for iron deficiency with best sensitivity/specificity 1, 3
• >150 μg/L: Makes absolute iron deficiency unlikely, even with inflammation 1

Step 2: Add Transferrin Saturation (TSAT) and TIBC

When ferritin is equivocal or inflammation is present, use TSAT and TIBC to refine diagnosis 1:

Iron Deficiency Anemia:

• Ferritin: Low (<30 μg/L)
• TSAT: Low (<16%)
• TIBC: High (elevated)
• Serum Iron: Low 1, 4

Anemia of Chronic Disease:

• Ferritin: Normal to high (can be 30-100 μg/L with inflammation)
• TSAT: Low (<20%)
• TIBC: Low (decreased)
• Serum Iron: Low 1, 4

Thalassemia:

• Ferritin: Normal
• TSAT: Normal to high
• TIBC: Normal
• Serum Iron: Normal to elevated 1, 4
• Key distinguishing feature: MCV disproportionately low relative to degree of anemia, with RDW normal or near-normal 3, 5

Sideroblastic Anemia:

• Ferritin: Normal to high (often elevated)
• TSAT: High (>45%)
• TIBC: Normal to low
• Serum Iron: Normal to elevated 1

Critical Pitfalls to Avoid

Ferritin in Inflammatory States

Ferritin is an acute phase reactant and can be falsely elevated in inflammation. 1 In patients with active inflammatory bowel disease, chronic infections, or malignancy:

• Ferritin up to 100 μg/L may still represent iron deficiency 1
• Use TSAT <20% with ferritin 30-100 μg/L to identify functional iron deficiency in inflammatory states 1
• The hepcidin/TSAT ratio is elevated in anemia of chronic disease 1

Don't Rely on MCV Alone

MCV loses sensitivity when combined deficiencies exist. 1 Microcytosis and macrocytosis can neutralize each other (e.g., iron deficiency + B12 deficiency), resulting in normal MCV with elevated RDW 1. Always check RDW:

• RDW >14%: Suggests iron deficiency anemia 3, 5
• RDW normal: More consistent with thalassemia trait 3, 4

Exclude Thalassemia Before Iron Supplementation

Order hemoglobin electrophoresis if microcytosis persists with normal iron studies, especially in patients of Mediterranean, African, Middle Eastern, or Southeast Asian descent. 1, 3 Beta-thalassemia trait shows elevated hemoglobin A2 levels 4. Inappropriate iron therapy in thalassemia can cause iron overload 1.

Special Considerations for Rare Genetic Disorders

When common causes are excluded and the patient has 1:

• Very low TSAT with low-normal ferritin (>20 μg/L): Consider IRIDA (iron-refractory iron deficiency anemia due to TMPRSS6 mutations) 1
• High TSAT with elevated ferritin: Consider hereditary sideroblastic anemias (ALAS2, SLC25A38 defects) 1
• Family history of refractory anemia: Pursue genetic testing 1, 3

These patients typically present in childhood, fail to respond to oral iron, and may require intravenous iron or pyridoxine supplementation 1, 3.

Practical Summary Table

Condition	Ferritin	TSAT	TIBC	Serum Iron	RDW
Iron Deficiency	Low (<30)	Low (<16%)	High	Low	High (>14%) [1,3]
Anemia of Chronic Disease	Normal-High (30-100+)	Low (<20%)	Low	Low	Normal-High [1,4]
Thalassemia	Normal	Normal-High	Normal	Normal-High	Normal [1,4]
Sideroblastic	Normal-High	High (>45%)	Normal-Low	Normal-High	Variable [1]