What laboratory studies should be ordered to evaluate a patient with thalassemia trait?

Laboratory Evaluation for Thalassemia Trait

For a patient with suspected thalassemia trait, order a complete blood count with red cell indices (MCV, MCH, RDW), peripheral blood smear, serum ferritin, and hemoglobin electrophoresis with HbA2 quantification—but critically, correct any iron deficiency before measuring HbA2, as iron deficiency falsely lowers HbA2 levels and can mask beta-thalassemia trait. 1

Initial Red Cell Indices Analysis

The diagnostic approach begins with careful interpretation of the CBC:

• MCV (Mean Corpuscular Volume): Typically low, often below the 5th percentile for age in thalassemia trait 1
• RBC count: Normal or elevated in thalassemia trait (contrast with iron deficiency where it's typically low) 1
• RDW (Red Cell Distribution Width): This is the critical differentiating factor—RDW ≤14.0% suggests thalassemia trait, while RDW >14.0% suggests iron deficiency anemia 1

The Mentzer index (MCV/RBC count) provides the highest reliability for distinguishing beta-thalassemia trait from iron deficiency, with 98.7% sensitivity and 82.3% specificity 2. A Mentzer index <13 suggests thalassemia trait, while >13 suggests iron deficiency 2.

Essential Iron Studies

Serum ferritin must be measured to rule out concurrent iron deficiency, which can mask thalassemia trait characteristics 1. This is a common diagnostic pitfall—iron deficiency and thalassemia trait can coexist, and iron deficiency will:

• Falsely lower HbA2 levels in beta-thalassemia trait 1
• Obscure the typical laboratory findings of thalassemia trait 1

If ferritin is below the reference range, provide iron replacement therapy before proceeding with definitive hemoglobin analysis 1. In pregnant women with suspected thalassemia trait and concurrent iron deficiency, this becomes particularly important, as mild anemia unresponsive to iron therapy may indicate thalassemia minor or sickle cell trait 3.

Peripheral Blood Smear

Peripheral blood smear examination is essential to confirm the size, shape, and color of red blood cells 1. In thalassemia trait, expect to see microcytic hypochromic erythrocytes with characteristic morphology 4.

Hemoglobin Electrophoresis with HbA2 Quantification

HbA2 measurement is the definitive test for beta-thalassemia trait, but timing is critical 1:

• Only perform after iron deficiency has been corrected, as iron deficiency falsely lowers HbA2 levels 1
• In beta-thalassemia trait, HbA2 is typically elevated (>3.5%) 5
• For beta-0-thalassemia trait, MCH ≤20.5 pg has 85% sensitivity and 90% specificity 4

Additional Considerations

Reticulocyte count should be obtained to distinguish between decreased production versus increased destruction or loss of red blood cells 1. This helps differentiate thalassemia trait from hemolytic processes.

Alpha-thalassemia trait presents differently—these individuals are typically asymptomatic and may show normal hemoglobin electrophoresis, as HbA2 is not elevated 6. Diagnosis may require DNA analysis if clinical suspicion is high despite normal HbA2.

Common Diagnostic Pitfalls to Avoid

1. Never measure HbA2 in the presence of iron deficiency—this is the single most important caveat 1
2. Lead poisoning, anemia of chronic inflammation, and sickle cell trait can present with similar red cell indices and must be excluded 1
3. In patients of African, Mediterranean, or Southeast Asian ancestry with mild anemia unresponsive to iron therapy, strongly consider thalassemia minor or sickle cell trait 3

Clinical Context

Persons with thalassemia trait are asymptomatic and have a normal life expectancy 6. However, genetic counseling should be offered, particularly for couples where both partners may carry thalassemia genes, as offspring could have thalassemia major 6.