RDW (red cell distribution width)

RDW (red cell distribution width) measures how much your red blood cells vary in size. Lab machines report it as a percentage, and the normal range is about 11.5 to 14.5%. A high RDW means your red cells are an uneven mix of sizes. That can come from low iron, B12, or folate, from red-cell breakdown (hemolysis), a transfusion, or faulty red-cell production. Here is the interesting part. Beyond diagnosing anemia, a higher RDW reliably predicts death from all causes and from heart disease, even when it sits inside the normal range. Why? Proposed reasons include chronic inflammation, oxidative stress, poor nutrition, and weaker bone-marrow function, which all disturb red-cell production. So RDW is increasingly seen as a cheap, simple marker of biological aging and physiological reserve.

Last reviewed: June 22, 2026

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