How should I evaluate a patient for thalassemia?

How to Test for Thalassemia

Begin with a Complete Blood Count (CBC) with red cell indices, followed by hemoglobin electrophoresis with HbA2 quantification after excluding iron deficiency. 1, 2

Initial Laboratory Assessment

The diagnostic approach starts with specific red cell parameters that distinguish thalassemia from other microcytic anemias:

• Mean Corpuscular Volume (MCV): Markedly reduced to 60-70 fL in beta-thalassemia trait, with slight to moderate reduction in alpha-thalassemia trait 2, 3
• Mean Corpuscular Hemoglobin (MCH): Reduced to 19-23 pg in beta-thalassemia trait; MCH <26 pg provides 99% sensitivity for detecting beta-thalassemia trait 2, 3
• Red Blood Cell (RBC) count: Normal or elevated in thalassemia trait, which contrasts with the typically low RBC count in iron deficiency anemia 1, 4
• Red Blood Cell Distribution Width (RDW): RDW ≤14.0% suggests thalassemia trait, while RDW >14.0% suggests iron deficiency anemia 1

Critical Step: Exclude Iron Deficiency First

You must measure serum ferritin and correct any iron deficiency before proceeding with hemoglobin analysis, as iron deficiency falsely lowers HbA2 levels and masks thalassemia trait characteristics. 1, 2

• If ferritin is below the reference range, provide iron replacement therapy before final assessment 1
• Calculate iron/total iron-binding capacity/saturation index to confirm iron status 2

Peripheral Blood Smear

Perform visual examination of the peripheral blood smear to confirm microcytic, hypochromic red blood cells and assess RBC morphology 5, 1

Hemoglobin Electrophoresis and Quantification

HbA2 determination is the most decisive test for beta-thalassemia carrier detection:

• Beta-thalassemia trait: HbA2 >3.5% confirms the diagnosis 2, 6
• Alpha-thalassemia trait: HbA2 can be lower than normal, which assumes significant diagnostic value only after iron deficiency is excluded 2
• This test must be performed after iron deficiency correction, as concurrent iron deficiency can falsely lower HbA2 levels 1, 2

Molecular Analysis

• Beta-thalassemia: Molecular analysis is not required to confirm carrier status, as HbA2 elevation is diagnostic 2
• Alpha-thalassemia: DNA testing is necessary to confirm carrier status, as hemoglobin electrophoresis findings are non-specific 2, 7
• Molecular diagnosis is essential to predict severe transfusion-dependent thalassemia major versus milder forms 2

Discriminant Indices (Screening Tools)

While several mathematical indices exist to differentiate thalassemia from iron deficiency, use them cautiously:

• Mentzer Index (MI): MCV/RBC count; MI <13 suggests thalassemia trait (MI <12 in 75% of cases, <15 in 97% of beta-thalassemia trait cases) 3
• These formulas should be used consciously, as the only truly discriminating parameter is the RBC count 2
• Discriminant analysis using MCH, RBC count, MCV, and RDW can differentiate diagnoses with 80.4% accuracy 4

Common Pitfalls to Avoid

• Concurrent iron deficiency is the most important pitfall—it masks typical thalassemia findings, particularly HbA2 elevation in beta-thalassemia trait 1, 2
• Lead poisoning, anemia of chronic inflammation, and sickle cell trait can present with similar red cell indices and must be excluded 1
• Delta-thalassemia defects can disturb HbA2 determination in beta-thalassemia carrier detection 2

Special Populations

Pregnant women: If suspected thalassemia trait with concurrent iron deficiency shows persistent mild anemia despite adequate iron therapy, evaluate for beta-thalassemia minor or sickle cell trait 1

Ancestry-based screening: Individuals of African, Mediterranean, or Southeast Asian ancestry with mild anemia unresponsive to iron supplementation should be assessed for beta-thalassemia minor or sickle cell trait 1

Prenatal Diagnosis

For confirmed carriers planning pregnancy:

• Prenatal diagnosis through fetal blood sampling to detect abnormal hemoglobin 8
• DNA testing for common deletions or point mutations from amniocentesis or chorionic villus sampling 8
• Parental MCV screening to identify potential carriers 8