Anisocytosis
Summary
Anisocytosis refers to abnormal variation in the size of red blood cells (RBCs), observed on peripheral blood smear. It is quantified by the RBC distribution width (RDW) on a CBC and is a nonspecific finding seen in many anemias. It indicates heterogeneity in RBC population rather than a specific diagnosis.
Detail
Anisocytosis is a morphological term describing variability in erythrocyte size, as opposed to poikilocytosis which refers to variability in shape. It is identified on peripheral blood smear review and correlates with an elevated RDW (>14.5%) on automated CBC analysis.
Clinical significance: Anisocytosis is nonspecific but useful when combined with mean corpuscular volume (MCV) to narrow the differential diagnosis of anemia. For example: - Iron deficiency anemia: microcytic cells with high RDW (significant anisocytosis) due to mixed population of normal and small cells, especially early in the disease course. - Vitamin B12/folate deficiency: macrocytic anemia with elevated RDW due to variable-sized macro-ovalocytes. - Thalassemia trait: microcytic but typically normal RDW (uniform small cells), helping distinguish it from iron deficiency. - Mixed deficiency anemias (e.g., combined iron and B12 deficiency) often show a dimorphic population with high RDW and normal/near-normal MCV. - Post-transfusion or post-hemorrhage states can also show anisocytosis due to mixing of new reticulocytes (larger) with older RBCs.
Anisocytosis does not localize to a specific cause but prompts further workup (iron studies, B12/folate levels, hemoglobin electrophoresis, reticulocyte count) to determine etiology. It is a key teaching point for Step 1 to link RDW with MCV in anemia algorithms.
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Hematology: Basic Principles and Practice (Hoffman)
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