RDW (Red Blood Cell Deviation Width) gives an idea of the percent variation in size of red blood cells. The larger the red blood cell size variation (anisocytosis), the higher the RDW value. RDW is interpreted in light of other complete blood count indices, including the MCV. In general, a high RDW suggests iron deficiency anemia, vitamin B12 or folic acid deficiency, or bone marrow disease (myelodysplastic syndrome). Anemia can also be found with a normal RDW in individuals who carry an abnormal hemoglobin production gene (alpha or beta thalassemia trait).