The red blood cell distribution width (RDW or RCDW) is a measure of the variation of red blood cell (RBC) width that is reported as part of a standard complete blood count. Usually red blood cells are a standard size of about 6–8 μm. Certain disorders, however, cause a significant variation in cell size. Higher RDW values indicate greater variation in size. Normal reference range in human red blood cells is 11–15%. If anemia is observed, RDW test results are often used together with mean corpuscular volume (MCV) results to determine the possible causes of the anemia. It is mainly used to differentiate an anemia of mixed causes from an anemia of a single cause. Vitamin B12 deficiency produces a macrocytic anemia (large cell anemia) with a normal RDW. However, iron deficiency anemia initially presents with a varied size distribution of red blood cells, and as such shows an increased RDW. In the case of a mixed iron and B12 deficiency, there will normally be a mix of both large cells and small cells, causing the RDW to be elevated. An elevated RDW (red blood cells of unequal sizes
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