Dimorphic anemia meaning
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dimorphic anemia definition causes peripheral smear
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Here is a complete answer on dimorphic anemia:
Dimorphic Anemia
Definition
Dimorphic anemia is the presence of two distinct populations of red blood cells (RBCs) on a peripheral blood smear - classically a mixture of microcytic hypochromic cells alongside normocytic (or macrocytic) cells. The word "dimorphic" simply means "two forms."
The blood film shows marked anisocytosis (variation in size) and anisochromia (variation in hemoglobin content/color). Because the MCV is calculated as an average, it may appear deceptively normal or only mildly abnormal - hiding the true mixed picture. The RDW (Red Cell Distribution Width) is elevated, which is an important clue.
Dimorphic anemia (Fig 31.13): Anisocytosis and anisochromia with microcytic hypochromic cells, normocytic cells, and a few macrocytes (1000x) - Henry's Clinical Diagnosis and Management by Laboratory Methods
Causes
| Cause | Mechanism |
|---|---|
| Sideroblastic anemia (most classic) | Defective heme synthesis produces a mixed population of hypochromic microcytes + normocytic/macrocytic cells. Characteristic ringed sideroblasts seen on bone marrow. Both acquired (MDS) and congenital forms. |
| Iron deficiency responding to iron therapy | After iron supplementation begins, new normochromic cells emerge alongside old hypochromic cells - two populations coexist for weeks |
| Combined iron + B12/folate deficiency | Iron deficiency pushes MCV down; B12/folate deficiency pushes MCV up; peripheral smear shows both microcytes and macrocytes. Classic example: atrophic gastritis causing both blood loss (iron deficiency) and B12 malabsorption. |
| Post-transfusion | Transfused normal donor cells mix with the patient's own hypochromic microcytic cells |
| Lead poisoning | Impairs heme synthesis similarly to sideroblastic anemia |
Key Lab Features
- Peripheral smear: Two RBC populations - small pale cells + larger normal/macro cells
- MCV: May be falsely normal (averaging effect) - do not rely on it alone
- RDW: Elevated (key indirect indicator of two populations)
- Serum ferritin, B12, folate: Order all three to identify the underlying deficiency
- Bone marrow: Ringed sideroblasts (>15% of erythroblasts) confirm sideroblastic anemia
Classic Associations to Remember
- Sideroblastic anemia = the textbook cause of dimorphic smear
- Normal/high MCV + low MCV together = think combined deficiency (B12/folate + iron)
- Weeks after starting iron = transient dimorphic picture as old cells are replaced
- Post-transfusion = temporary mixed picture
Sources: Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 636-637; Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine 22E; Rosen's Emergency Medicine
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