What is the likely diagnosis and treatment for a patient with hypochromic microcytic anemia, characterized by low Mean Corpuscular Hemoglobin Concentration (MCHC) and low Red Cell Distribution Width (RDW)?

Low MCHC and Low RDW: Diagnostic Approach

A low MCHC combined with a low RDW (≤14.0%) in the setting of microcytic anemia strongly suggests thalassemia minor rather than iron deficiency anemia. 1

Diagnostic Algorithm

Initial Interpretation of Red Cell Indices

• Low MCV with low RDW (≤14.0%) indicates thalassemia minor, while low MCV with high RDW (>14.0%) indicates iron deficiency anemia, according to the Centers for Disease Control and Prevention 1
• Low MCHC combined with low MCHC strongly suggests hypochromic anemia, but the low RDW is the critical distinguishing feature that points away from iron deficiency 2
• Iron deficiency anemia typically causes greater variation in red blood cell size (elevated RDW) than thalassemia minor, which produces uniformly small cells 1

Confirmatory Testing Required

Despite the pattern suggesting thalassemia, you must confirm iron status before concluding the diagnosis:

• Measure serum ferritin first - this is the most powerful test for iron deficiency, with levels <12 μg/dL diagnostic of iron deficiency 1
• If ferritin is <15 μg/L, iron stores are absent; if <30 μg/L, iron stores are low 2
• Order hemoglobin electrophoresis if iron studies are normal, particularly given the microcytosis with normal/low RDW pattern 2
• Consider transferrin saturation if ferritin appears falsely elevated due to inflammation (ferritin can be elevated up to 100 μg/L in inflammatory states while iron deficiency still exists) 2

Differential Diagnosis Considerations

The combination of low MCHC and low RDW narrows your differential significantly:

• Thalassemia minor (most likely): Characterized by low MCV, low RDW, normal or elevated red blood cell count, and normal iron studies 1, 3
• Early iron deficiency (less likely): Before significant anisocytosis develops, though this is uncommon 4
• Hemoglobin E disorders: Present similarly to thalassemia with microcytosis and normal iron studies 4

Critical Pitfalls to Avoid

• Do not assume all microcytic anemia is iron deficiency - the low RDW specifically argues against this diagnosis 2
• Do not start empiric iron therapy without confirming iron deficiency - treating thalassemia patients with iron can lead to iron overload 1
• Do not overlook combined deficiencies - a patient can have both thalassemia trait and concurrent iron deficiency, which may be recognized by checking both hemoglobin electrophoresis and iron studies 2
• Consider ethnic background - thalassemias and hemoglobinopathies are more common in certain ethnic groups (Mediterranean, Southeast Asian, African descent) 1

When Iron Deficiency is Confirmed Despite Low RDW

If iron studies confirm deficiency (ferritin <30 μg/L, transferrin saturation <20%), this represents early iron deficiency before significant anisocytosis develops 2:

• First-line treatment: Ferrous sulfate 200 mg three times daily for at least three months after correction of anemia 2
• Expected response: Hemoglobin should rise ≥10 g/L within 2 weeks if iron deficiency is the cause 2
• Investigate the source of iron loss in all adults, particularly men with Hb <110 g/L or non-menstruating women with Hb <100 g/L, focusing on gastrointestinal blood loss 2

When Thalassemia is Confirmed

• No iron supplementation is needed unless concurrent iron deficiency is documented 1
• Genetic counseling should be offered, particularly for reproductive planning 3
• Monitor for complications of chronic hemolysis if symptomatic 3