Red blood cell distribution width
Red blood cell distribution width (RDW) is a measure from a standard blood test that shows how much red blood cell sizes vary in a sample. Red cells typically range in size, and a higher RDW means more variation (anisocytosis).
RDW-CV is normally about 11.5% to 15.4%. RDW-SD is another way to report the variation and is the width of the size distribution at a certain height on a histogram; it’s not affected by the average cell size (MCV).
Why RDW matters:
- If anemia is present, doctors compare RDW with MCV (average RBC size) to help figure out the cause.
- Iron deficiency anemia usually shows high RDW (large variation as new small cells appear with ongoing loss).
- Vitamin B12 or folate deficiency often raises RDW in many cases (macrocytic anemia).
- Some conditions, like thalassemia, can have normal RDW even when anemia is present; other tests (like hemoglobin electrophoresis) are then needed.
- Anemia of chronic disease, some hereditary conditions, acute blood loss, and aplastic anemia may have a normal RDW.
RDW can also be higher if there are abnormal cell shapes or cell fragments.
In short: RDW helps doctors distinguish between types of anemia, but it must be interpreted alongside other blood test results.
This page was last edited on 2 February 2026, at 11:14 (CET).