Differentiating Iron Deficiency Anemia from Alpha Thalassemia
The most reliable approach combines RDW with serum ferritin: an elevated RDW >14% with low ferritin (<30 μg/L) confirms iron deficiency, while RDW ≤14% with normal ferritin (>30 μg/L) indicates thalassemia trait. 1, 2
Primary Diagnostic Algorithm
Step 1: Order initial laboratory tests
- Obtain serum ferritin, complete blood count with RDW, and transferrin saturation simultaneously 1, 3
- These three tests provide the foundation for distinguishing between the two conditions 1
Step 2: Apply the ferritin-RDW rule
- Ferritin <30 μg/L + RDW >14% → Iron deficiency anemia; proceed to investigate bleeding sources 1, 2
- Ferritin >30 μg/L + RDW ≤14% → Thalassemia trait; order hemoglobin electrophoresis for confirmation 1, 2
- A ferritin cut-off of 45 μg/L provides optimal sensitivity and specificity in clinical practice 1, 3
Step 3: Use transferrin saturation when ferritin is equivocal (30-100 μg/L)
- TSAT <16-20% confirms iron deficiency even when ferritin appears borderline 1, 2
- This is particularly important because ferritin is an acute-phase reactant and can be falsely elevated by inflammation, infection, malignancy, or liver disease 1
- Measure C-reactive protein concurrently to identify inflammatory states 1
Key Distinguishing Features
Red cell distribution width (RDW)
- Iron deficiency produces RDW >14% because it creates a mixed population of older normal-sized cells and newer microcytic cells 1, 2
- Thalassemia trait typically shows RDW ≤14% because red cells are uniformly small 1, 2
- However, approximately 50% of thalassemia cases can exhibit elevated RDW, so concurrent low serum iron and high RDW together make iron deficiency more likely 1
Red blood cell count
- Thalassemia trait shows increased RBC count (>5.0 × 10¹²/L) despite microcytosis 4
- Iron deficiency typically has normal or decreased RBC count 4
Mean corpuscular hemoglobin concentration (MCHC)
- MCHC is more reliable than MCV because it is less affected by specimen storage conditions 1
- Both conditions show reduced MCHC, but the pattern differs when combined with RBC count 4, 5
When to Order Hemoglobin Electrophoresis
Do NOT order electrophoresis as a first-line test 1
Order hemoglobin electrophoresis only when:
- Microcytosis persists despite normal iron studies (ferritin >30 μg/L and TSAT >20%) 1
- MCV is disproportionately low relative to the severity of anemia 1, 2
- Patient belongs to high-risk ethnic group (African, Mediterranean, or Southeast Asian ancestry) 1
- Anemia fails to improve after 4 weeks of adequate oral iron therapy despite good compliance 1
Advanced Discriminatory Parameters (if available)
Reticulocyte hemoglobin equivalent (Ret-He)
- Ret-He is significantly reduced in both alpha and beta thalassemia compared to iron deficiency 6
- Iron deficiency shows increased Ret-He values 6
Immature reticulocyte fraction (IRF)
- IRF is significantly higher in iron deficiency compared to thalassemia 6
Hemoglobin distribution width (HDW)
- Beta-thalassemia shows increased HDW due to broadening of the hemoglobin concentration histogram 4
Critical Pitfalls to Avoid
- Do not rely on serum iron alone—it shows considerable day-to-day variability and overlaps between the two conditions 1
- Do not use hemoglobin electrophoresis as a screening test—it is costly and unnecessary when iron studies are abnormal 1
- Do not ignore inflammation—ferritin up to 100 μg/L may still indicate iron deficiency when CRP is elevated; use TSAT <20% to confirm 1
- Do not overlook combined deficiencies—iron deficiency can coexist with folate or B12 deficiency, which may also elevate RDW 1, 2
- Do not assume RDW ≤14% excludes iron deficiency—about half of thalassemia cases show elevated RDW, so interpret RDW alongside ferritin and TSAT 1