Which laboratory finding best distinguishes α-thalassemia trait (α-thalassemia 2) from β-thalassemia trait in a 25-year-old Indian woman with microcytic hypochromic anemia?

Question

Accepted Answer

D. Elevated HbA2 (>3.5%) on hemoglobin electrophoresis. ## α-Thalassemia Trait vs. β-Thalassemia Trait **Key Point:** **Elevated HbA2 (>3.5%)** is the single best discriminator. β-thalassemia trait shows HbA2 elevation; α-thalassemia trait does not. ### Pathophysiology Basis **β-Thalassemia trait:** - Reduced β-globin chain synthesis → relative excess of α and δ chains. - δ chains combine with excess α chains to form **HbA2** (α~2~δ~2~). - Compensatory increase in δ-globin gene expression. - HbA2 rises to **3.5–7%** (normal <3.5%). **α-Thalassemia trait (2-gene deletion):** - Reduced α-globin chain synthesis → relative excess of β and γ chains. - β chains form β~4~ tetramers (HbH) in RBCs; γ chains form γ~4~ (Hb Bart's) in neonates. - No δ-chain excess, so **HbA2 remains normal (<3.5%)**. ### Comparison Table | Feature | α-Thalassemia Trait | β-Thalassemia Trait | |---------|-------------------|-------------------| | **HbA2 level** | Normal (<3.5%) | **Elevated (3.5–7%)** ✓ | | **HbF level** | Normal | Mildly elevated (2–5%) | | **Heinz bodies** | **Present** (HbH inclusions) | Absent | | **RDW** | Normal or mildly elevated | Elevated | | **MCV** | Low (60–70 fL) | Low (60–70 fL) | | **Serum iron/ferritin** | Normal | Normal | | **Osmotic fragility** | Normal or increased | **Decreased** | **High-Yield:** **HbA2 electrophoresis is the gold standard** to distinguish these two trait forms. It is quick, inexpensive, and universally available. **Mnemonic:** **"A2 in Beta"** — HbA2 rises in β-thalassemia trait (because δ chains increase), but **not in α-thalassemia trait** (because δ-globin genes are normal). **Clinical Pearl:** In populations with high carrier frequency (Mediterranean, Middle Eastern, Asian), HbA2 measurement is routine screening. A microcytic anemia with elevated HbA2 strongly suggests β-thalassemia trait; normal HbA2 in microcytosis suggests α-thalassemia trait or iron deficiency.

Answer

A. Presence of Heinz bodies on supravital staining

Answer

B. Elevated red cell distribution width (RDW)

Answer

C. Microcytic indices with normal or low MCV

Which laboratory finding best distinguishes α-thalassemia trait (α-thalassemia 2) from β-thalassemia trait in a 25-year-old Indian woman with microcytic hypochromic anemia?

Explanation

α-Thalassemia Trait vs. β-Thalassemia Trait

Pathophysiology Basis

Comparison Table

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Feature	α-Thalassemia Trait	β-Thalassemia Trait
HbA2 level	Normal (<3.5%)	Elevated (3.5–7%) ✓
HbF level	Normal	Mildly elevated (2–5%)
Heinz bodies	Present (HbH inclusions)	Absent
RDW	Normal or mildly elevated	Elevated
MCV	Low (60–70 fL)	Low (60–70 fL)
Serum iron/ferritin	Normal	Normal
Osmotic fragility	Normal or increased	Decreased