RDW
Red cell distribution width (RDW) measures the variation in red blood cell size (anisocytosis). A normal RDW means RBCs are uniformly sized; a high RDW means a mix of small and large cells. It is calculated as the coefficient of variation of red cell volume.
Why it matters: RDW is a powerful but underappreciated marker. In anemia workup, elevated RDW distinguishes iron deficiency (high RDW) from thalassemia (normal RDW). Beyond hematology, elevated RDW independently predicts cardiovascular mortality, heart failure outcomes, and all-cause mortality — even within the normal hemoglobin range. It is a PhenoAge biomarker.
May increase with:
May decrease with:
What is RDW?
Red cell distribution width (RDW) measures the variation in red blood cell size (anisocytosis). A normal RDW means RBCs are uniformly sized; a high RDW means a mix of small and large cells. It is calculated as the coefficient of variation of red cell volume.
What might a high or low RDW mean?
RDW is a powerful but underappreciated marker. In anemia workup, elevated RDW distinguishes iron deficiency (high RDW) from thalassemia (normal RDW). Beyond hematology, elevated RDW independently predicts cardiovascular mortality, heart failure outcomes, and all-cause mortality — even within the normal hemoglobin range. It is a PhenoAge biomarker.
What is the typical reference range for RDW?
The general-population reference range shown here is 11.5 – 14.5 %. Reference ranges describe the general population and are not a personal target — discuss your results with your physician.
What can affect RDW?
It may be higher with: Iron deficiency (new small RBCs mixed with old normal ones), B12/folate deficiency, Mixed anemias, Recent blood transfusion, Iron supplementation during recovery, Chronic liver disease, Cardiovascular disease, Chronic inflammation. It may be lower with: Correction of the underlying deficiency, Uniform RBC production (normal state), Thalassemia trait (uniformly small cells = normal RDW).