Red blood cell distribution width
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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 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.
Mathematically the RDW is calculated with the following formula:
- RDW = (Standard deviation of red cell volume ÷ mean cell volume) × 100
RDW calculations can give a more objective picture of the degree of variability of cell size known as anisocytosis.
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