Red blood cell distribution width
Red blood cell distribution width (RDW), as well as various types thereof (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. Red blood cells have an average volume of 80-100 femtoliters, but individual cell volumes vary even in healthy blood. Certain disorders, however, cause a significantly increased variation in cell size. Higher RDW values indicate greater variation in size. Normal reference range of RDW-CV in human red blood cells is  11.5–15.4%. 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. 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. 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. Calculations. The "width" in RDW-CV is sometimes thought  to be "misleading", since it in fact is a measure of deviation of the "volume" of RBCs, and not directly the diameter. However, "width" refers to the width of the volume curve (distribution width, here presented as the coefficient of variation, or CV), not the width of the cells. Thus, it is a reasonably accurate term. Mathematically, the RDW-CV is calculated with the following formula: Pathological implications. Normal RDW. Anemia in the presence of a normal RDW may suggest thalassemia. A low Mentzer Index, calculated from CBC data [MCV/RBC < 13], may suggest this disorder but a hemoglobin  electrophoresis would be diagnostic. Anemia of chronic diseases show normal RDW. High RDW. High RDW may be a result of the presence of fragments, groups of agglutination, and/or abnormal shape of red blood cells. By severity, elevated RDW can be classified as follows: