The red blood cell distribution width, or RDW, is a measure of the variation of red blood cell volume 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 numbers indicate greater variation in size. Normal range 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 and other microcytic anemias in children. It may denote hereditary spherocytosis

Mathematically the RDW is calculated with the following formula:

RDW = (Standard deviation of red cell volume ÷ mean cell volume) × 100

Hematology