<P> Red blood cell distribution width (RDW or RDW - CV or RCDW and RDW - SD) is a measure of the range of variation of red blood cell (RBC) volume that is reported as part of a standard complete blood count . Usually red blood cells are a standard size of about 6 - 8 μm in diameter . Certain disorders, however, cause a significant variation in cell size . Higher RDW values indicate greater variation in size . Normal reference range of RDW - CV in human red blood cells is 11.5 - 14.5% . 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 . </P> <P> Deficiencies of Vitamin B or folate produce a macrocytic anemia (large cell anemia) in which the RDW is elevated in roughly two - thirds of all cases . However, a varied size distribution of red blood cells is a hallmark of iron deficiency anemia, and as such shows an increased RDW in virtually all cases . In the case of both iron and B deficiencies, 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) is known as anisocytosis . </P>

What is an rdw in a blood test