RDW (red cell distribution width)

Red cell distribution width (RDW) is a quantitative measure of the variability in erythrocyte volume (anisocytosis), reported as the coefficient of variation of the erythrocyte volume distribution from automated hematology analyzers; the normal range is approximately 11.5–14.5%. Elevated RDW reflects heterogeneous red cell populations and arises from iron, B12, or folate deficiency, hemolysis, blood transfusion, or ineffective erythropoiesis. Independent of its role in anemia workup, higher RDW is a robust, reproducible predictor of all-cause and cardiovascular mortality in community populations, with associations observed even within the reference range. Proposed mechanisms include chronic inflammation, oxidative stress, nutritional deficiency, and impaired bone marrow function, all of which impair erythropoietic homeostasis. RDW is increasingly recognized as a simple, inexpensive marker of biological aging and physiological reserve.