Erythrocyte distribution width is a parameter that refers to the size and heterogeneity of the red blood cell volume, which is expressed by the coefficient of variation of the red blood cell volume. An increased erythrocyte width of distribution is indicative of heterogeneity in the size of red blood cells in the body. This item is used clinically for the classification of anemia as well as for differential diagnosis and some early diagnosis, the most common disease being iron deficiency anemia. In Hainan, there is a high prevalence of anemia called thalassemia, which has a normal width of red blood cell distribution. Iron deficiency anemia is also a small cell hypochromic anemia with very heterogeneous red blood cell volume size and increased RDW, and the two types of anemia can be identified by the red blood cell distribution width. Whether the cell volume size is truly inhomogeneous also does not depend entirely on the RDW value on the testing instrument. When the RDW value exceeds 22%, a manual smear microscopy is performed to confirm the cell size inhomogeneity.