What tests are used to diagnose beta (β) thalassemia?

Diagnostic Tests for Beta (β) Thalassemia

Beta-thalassemia diagnosis requires a systematic laboratory approach starting with complete blood count (CBC) showing microcytic anemia with low MCV (typically <80 fL), followed by hemoglobin electrophoresis or high-performance liquid chromatography (HPLC) to measure HbA2 levels, and confirmed with genetic testing when needed. 1, 2, 3

Initial Laboratory Screening

Complete Blood Count (CBC) Findings

• Mean Corpuscular Volume (MCV) is characteristically low, typically below 80 fL and often below the 5th percentile for age 1, 4
• Red Blood Cell (RBC) count is normal or elevated, which distinguishes thalassemia trait from iron deficiency anemia where RBC count is typically low 1, 4
• Red Blood Cell Distribution Width (RDW) ≤14.0% suggests thalassemia trait, while RDW >14.0% suggests iron deficiency anemia 1
• Hemoglobin level is decreased in beta-thalassemia major and intermedia, but may be normal or only mildly decreased in beta-thalassemia trait 5, 2

Peripheral Blood Smear

• Examination reveals microcytic, hypochromic red blood cells with characteristic morphologic abnormalities 1, 6
• Reticulocyte count helps distinguish between decreased production versus increased destruction of red blood cells 1

Iron Status Assessment

Critical caveat: Iron deficiency must be ruled out or corrected before proceeding with definitive hemoglobin analysis, as concurrent iron deficiency can falsely lower HbA2 levels and mask beta-thalassemia trait. 1

• Serum ferritin should be measured to exclude iron deficiency 1
• If ferritin is below the reference range, provide iron replacement therapy before final thalassemia assessment 1
• Normal ferritin levels support thalassemia diagnosis over iron deficiency 4

Hemoglobin Analysis (Definitive Testing)

Hemoglobin Electrophoresis or HPLC

• HbA2 measurement is the key diagnostic test for beta-thalassemia trait 1, 2
• In beta-thalassemia trait (β-thalassemia minor), HbA2 is typically elevated (>3.5%) 2, 3
• HbF (fetal hemoglobin) may be mildly elevated in some cases 2, 3
• High-performance liquid chromatography (HPLC) is the preferred modern method for hemoglobin separation and quantification 3
• Mass spectrometry is an alternative advanced technique for detecting hemoglobin abnormalities 3

Newborn Screening Results

• In beta-thalassemia major (Sβ0-thalassemia when combined with sickle cell), newborn screening shows FS pattern 5
• In beta-thalassemia trait (Sβ+-thalassemia when combined with sickle cell), newborn screening shows FSA pattern 5

Genetic Testing (Confirmatory)

• DNA analysis identifies specific beta-globin gene mutations and is required for definitive diagnosis 2, 7, 3
• Genetic testing is particularly important for:
  • Confirming diagnosis when hemoglobin electrophoresis results are equivocal 3
  • Prenatal diagnosis and genetic counseling 7, 8, 3
  • Distinguishing between different beta-thalassemia subtypes 3
• Massively parallel sequencing (next-generation sequencing) enables more precise molecular diagnosis 3
• Beta-thalassemia is caused by point mutations rather than deletions (unlike alpha-thalassemia) 2, 8

Parental Screening for Prenatal Diagnosis

• Mean Corpuscular Volume (MCV) <80 fL in parents suggests thalassemia carrier status and warrants further evaluation 5
• DNA testing for specific beta-globin mutations should be performed if both parents are carriers 5
• Prenatal diagnosis can be performed via:
  • Chorionic villus sampling 8
  • Amniocentesis with genetic testing 5
  • Amniotic fluid alpha-fetoprotein (AFAFP) measurement 5

Diagnostic Algorithm Summary

1. Screen with CBC: Look for microcytic anemia (MCV <80 fL), normal/elevated RBC count, and RDW ≤14.0% 1, 4
2. Check iron status: Measure serum ferritin to exclude iron deficiency 1
3. Correct iron deficiency if present before proceeding 1
4. Perform hemoglobin analysis: HPLC or electrophoresis to measure HbA2 and HbF levels 1, 2, 3
5. Confirm with genetic testing: DNA analysis for beta-globin gene mutations when diagnosis needs confirmation or for genetic counseling 2, 7, 3

Common Pitfalls to Avoid

• Do not interpret HbA2 levels without first correcting iron deficiency, as iron deficiency falsely lowers HbA2 and can mask beta-thalassemia trait 1
• Lead poisoning, anemia of chronic inflammation, and sickle cell trait can present with similar microcytic indices and must be excluded 1
• Beta-thalassemia intermedia may present suddenly in adulthood despite benign past medical history, so maintain clinical suspicion even in adults with no prior diagnosis 6