What is dimorphic iron deficiency anemia?

What is Dimorphic Iron Deficiency Anemia

Dimorphic anemia refers to a condition where two distinct red blood cell populations coexist—typically both microcytic (small) and macrocytic (large) cells—resulting in a paradoxically normal mean corpuscular volume (MCV) that masks the underlying dual deficiency.

Definition and Pathophysiology

Dimorphic anemia occurs when iron deficiency (which causes microcytosis) exists simultaneously with another deficiency that causes macrocytosis, most commonly vitamin B12 or folate deficiency 1. The two abnormalities neutralize each other in the MCV calculation, potentially leading to misdiagnosis if only MCV is assessed 1.

Key Diagnostic Features

The hallmark laboratory finding is an elevated red cell distribution width (RDW), which indicates a wide size range of red blood cells and serves as a critical clue to the presence of combined deficiencies 1.

Laboratory Characteristics:

• Normal or near-normal MCV despite underlying deficiencies 1
• Elevated RDW (>14-15%), indicating heterogeneous red cell populations 1
• Low serum ferritin (<30 μg/L without inflammation, or <100 μg/L with inflammation) confirming iron deficiency 1
• Low vitamin B12 or folate levels confirming the macrocytic component 1
• Peripheral blood smear showing both microcytic and macrocytic red cells 1

Clinical Context and Common Causes

High-Risk Populations:

• Inflammatory bowel disease (IBD) patients who have both chronic blood loss (causing iron deficiency) and malabsorption (causing B12/folate deficiency) 1
• Post-gastrectomy patients with impaired iron absorption and intrinsic factor deficiency 1
• Patients on thiopurine therapy (azathioprine, 6-mercaptopurine) which causes macrocytosis, combined with GI blood loss 1
• Chronic alcoholics with poor nutrition and GI bleeding 1
• Celiac disease patients with malabsorption of multiple nutrients 1

Diagnostic Approach

When evaluating suspected dimorphic anemia:

1. Order complete blood count with RDW and reticulocyte count as initial screening 1
2. Check serum ferritin and transferrin saturation to assess iron stores 1
3. Measure vitamin B12 and folate levels to identify macrocytic component 1
4. Assess inflammatory markers (CRP, ESR) as ferritin thresholds differ with inflammation 1
5. Review peripheral blood smear to visualize the dimorphic red cell population 1

Critical Pitfall:

Do not rely on MCV alone to exclude iron deficiency or vitamin deficiency—a normal MCV with elevated RDW should prompt investigation for combined deficiencies 1. This is particularly important in IBD patients where both deficiencies commonly coexist 1.

Treatment Implications

Both deficiencies must be addressed simultaneously:

• Iron replacement (oral ferrous sulfate 325 mg daily or intravenous iron for malabsorption, chronic inflammation, or intolerance) 2, 3
• Vitamin B12 supplementation (typically 1000 mcg intramuscularly if pernicious anemia or malabsorption) 1
• Folate supplementation (1-5 mg daily orally) if folate deficiency is confirmed 1
• Treatment of underlying cause (IBD management, celiac disease treatment, cessation of alcohol) 1

Treating only one deficiency will result in incomplete hematologic response and persistent symptoms 1.