A 48-year-old female has a CBC showing a red cell distribution width (RDW) 18.5%. The normal adult reference range for RDW is 11.5–14.5%. What does this elevated RDW indicate?

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✅ D. Increased variation in red blood cell size (anisocytosis)
RDW (red cell distribution width) is a coefficient of variation that quantifies the range of RBC sizes within a sample. The normal range is 11.5–14.5%. A value above 14.5% indicates anisocytosis—a mixed population of small and large cells. Elevated RDW commonly appears in iron deficiency anemia, mixed nutritional deficiencies (iron + B12/folate), and the recovery phase after blood loss or treatment.

❌ A. Decreased hemoglobin per red blood cell
This describes a low MCH (mean corpuscular hemoglobin), expressed in picograms with a normal range of 27–33 pg. MCH measures mass per cell, not size variation.

❌ B. Increased average red blood cell size
This describes an elevated MCV (macrocytosis), measured in fL. MCV reports the AVERAGE cell size; RDW reports the VARIATION around that average. A patient can have normal MCV and still have high RDW.

❌ C. Decreased red blood cell count
A low RBC count is reported as RBC (×10^6/µL), not as a percent, and is a separate CBC parameter. RDW does not quantify the number of cells, only their size distribution.

:::pearl
MCV tells you the size. RDW tells you how UNIFORM those sizes are. Iron deficiency: high RDW (cells made with varying iron availability). Thalassemia trait: normal RDW (genetic defect affects every cell equally).
:::

RDW — Normal Range & Interpretation

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