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Red blood cell distribution width
Introduction
Overview
The red blood cell distribution width, or RDW, is a measure of the variation of red blood cell width that is reported as part of a standard complete blood count. Usually red blood cells are a standard size. Certain disorders, however, cause a significant variation in cell size. Higher RDW values indicate greater variation in size. Normal range in human red blood cells is 11 – 15%. If anemia is observed, RDW test results are often used together with MCV results to figure out what the cause of the anemia might be. It is mainly used to differentiate between iron deficiency anemia, in which RDW is elevated, and other microcytic anemias. It may denote hereditary spherocytosis. An elevated RDW, i.e. red blood cells of unequal sizes, is known as anisocytosis.
Mathematically the RDW is calculated with the following formula:
- RDW = (Standard deviation of red cell width ÷ mean cell width) × 100
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]