Red cell distribution width (RDW)

RDW measures the variation in red blood cell size, a property called anisocytosis. Healthy red cells are relatively uniform, which supports smooth circulation and predictable oxygen transport. A higher RDW reflects a broader range of cell sizes, often caused by iron deficiency, B12 or folate deficiency, marrow stress, or recent blood loss.Lower RDW is rare and usually carries little meaning on its own. RDW is automatically included in a complete blood count (CBC) and works best when interpreted alongside hemoglobin, mean cell volume (MCV), ferritin, and vitamin markers. ‍