<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> <P> An elevation in the RDW is not characteristic of all anemias . Anemia of chronic disease, hereditary spherocytosis, acute blood loss, aplastic anemia (anemia resulting from an inability of the bone marrow to produce red blood cells), and certain hereditary hemoglobinopathies (including some cases of thalassemia minor) may all present with a normal RDW . </P>

What does it mean when rdw is high