## 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.
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