Diagnosis: Thalassemia (Option B)
The diagnosis is thalassemia based on the combination of microcytic anemia (low MCV), low hemoglobin, normal RDW, nucleated RBCs, target cells, and hepatosplenomegaly in an infant. 1
Key Diagnostic Features Supporting Thalassemia
Blood Smear Findings
- Target cells are characteristic of thalassemia and hemoglobinopathies, not typically seen in iron deficiency anemia 2
- Nucleated RBCs (erythroblasts) indicate severe hemolysis or ineffective erythropoiesis, which is a hallmark of thalassemia major in infants 2
- The combination of target cells with nucleated RBCs strongly points toward a hemoglobinopathy rather than nutritional deficiency 2
Normal RDW: The Critical Distinguishing Feature
- Normal RDW (11) indicates a homogeneous population of microcytic cells, which is pathognomonic for thalassemia trait or disease 1, 3
- Iron deficiency anemia characteristically presents with elevated RDW (>14.0%) due to heterogeneous red cell populations (mixture of older normal-sized cells and newer microcytic cells) 4, 1
- Low MCV + Normal/Low RDW strongly suggests thalassemia rather than iron deficiency 1, 2
- While some thalassemia cases can have elevated RDW, the normal RDW in this clinical context makes iron deficiency highly unlikely 5
Hepatosplenomegaly
- Hepatosplenomegaly in an infant with microcytic anemia indicates extramedullary hematopoiesis, which occurs in severe thalassemia (beta-thalassemia major or HbH disease) 2
- This finding is not typical of iron deficiency anemia or hereditary spherocytosis in early infancy 2
Why Not the Other Options
Hereditary Spherocytosis (Option A) - Excluded
- Hereditary spherocytosis typically presents with normocytic or macrocytic anemia, not microcytic 6
- The blood smear would show spherocytes, not target cells 6
- RDW is usually elevated in hereditary spherocytosis due to heterogeneous cell populations 2
Iron Deficiency Anemia (Option C) - Excluded
- Iron deficiency virtually always presents with elevated RDW (>14.0%), not normal RDW 4, 1, 7
- Target cells are not characteristic of iron deficiency 2
- Nucleated RBCs are rare in iron deficiency unless extremely severe 3
- Hepatosplenomegaly is not a feature of iron deficiency anemia 2
Confirmatory Testing Recommended
- Hemoglobin electrophoresis is essential to confirm the specific type of thalassemia (beta-thalassemia major, HbH disease, or other variants) 1, 2
- Complete iron studies (ferritin, transferrin saturation) should be performed to exclude concurrent iron deficiency, though the normal RDW makes this unlikely 1, 2
- Do not initiate iron supplementation without confirming iron deficiency, as this can cause iron overload in thalassemia patients 1, 2
Clinical Pitfall to Avoid
Never assume iron deficiency based solely on microcytic anemia in an infant—the normal RDW makes iron deficiency extremely unlikely and should redirect evaluation toward hemoglobinopathies. 1, 4 The presence of hepatosplenomegaly, nucleated RBCs, and target cells in this context makes thalassemia the definitive diagnosis requiring genetic counseling and transfusion management rather than iron supplementation 2, 1