CBC Findings in Alpha Thalassemia Trait
The most characteristic CBC findings in alpha thalassemia trait include microcytosis (low MCV) with values often below 80 fL, mild or absent anemia, normal or elevated red blood cell count, and a normal red cell distribution width (RDW ≤14.0%) 1.
Laboratory Parameters in Alpha Thalassemia Trait
- Mean Corpuscular Volume (MCV): Typically low, with values often below the 5th percentile for age and frequently below 80 fL 1, 2
- Hemoglobin (Hb): Normal to mildly decreased, depending on the number of alpha genes affected 3
- Red Blood Cell (RBC) count: Normal or elevated, which helps differentiate from iron deficiency anemia where RBC count is typically low 1, 4
- Red Blood Cell Distribution Width (RDW): Usually normal (≤14.0%), which is an important differentiating factor from iron deficiency anemia where RDW is typically elevated (>14.0%) 1, 3
- Peripheral blood smear: Shows microcytic, hypochromic red cells, though less severe than in beta thalassemia trait 3
Differentiating Features Based on Number of Alpha Gene Deletions
Silent Carrier (one gene deletion, -α/αα):
- Minimal or no reduction in hemoglobin
- Borderline microcytosis
- Often clinically silent 5
Alpha Thalassemia Trait (two gene deletions, --/αα or -α/-α):
- Mild microcytosis
- Hemoglobin usually within normal range or slightly decreased
- No clinical symptoms 5
Key Distinguishing Features from Iron Deficiency Anemia
- In alpha thalassemia trait, the percentage of microcytes is typically higher than the percentage of hypochromic cells, whereas in iron deficiency anemia, the percentage of hypochromic cells exceeds the percentage of microcytes 4
- The ratio between the percentage of microcytes and hypochromic cells (microcytic-hypochromic ratio) >0.9 suggests thalassemia trait 4
- Normal ferritin levels in alpha thalassemia trait versus low ferritin in iron deficiency 1
Common Pitfalls in Diagnosis
- Concurrent iron deficiency can mask the typical laboratory findings of alpha thalassemia trait 1
- Alpha thalassemia trait may be overlooked in non-traditional ethnic groups, as it can occur in individuals of any ancestry, not just those of Southeast Asian, Mediterranean, or Middle Eastern descent 6
- When alpha thalassemia coexists with sickle cell trait, it can lead to decreased levels of HbS (below 35%) compared to the usual levels seen in sickle cell trait alone 7
Clinical Implications
- Alpha thalassemia trait is generally asymptomatic and does not require treatment 5
- Genetic counseling is important, particularly for reproductive planning, as certain combinations of alpha thalassemia mutations in parents can lead to more severe conditions in offspring, including Hemoglobin H disease or even Hemoglobin Bart's hydrops fetalis 2
- Alpha thalassemia is the most common cause of non-immune hydrops fetalis in Southeast Asian populations 8
Remember that definitive diagnosis of alpha thalassemia trait requires molecular genetic testing, as the CBC findings alone cannot distinguish between the various forms of alpha thalassemia or confirm the specific genetic mutations involved 3.