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%.[1] 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. The trait is Experimental Factor Ontology entry EFO_0009188 (Red cell distribution width). Also known as: Erythrocyte Distribution Width Measurement, RCDW, RDW, RDW-CV, RDW-SD, Red blood cell erythrocyte distribution width, Red cell distribution width (RDW) blood test.