Red blood cell distribution width (RDW) is a parameter that reflects the specificity of peripheral blood red blood cell volume and is important for the diagnosis of anemia. The normal adult erythrocyte distribution width is <0.15 (<15%). When the erythrocyte distribution width is higher than the normal value range it is called high erythrocyte distribution width, the higher the width, the different shapes and sizes of the sample blood erythrocytes. Erythrocyte distribution width is usually used together with mean red blood cell volume (MCV) for morphologic classification of anemia. If the mean erythrocyte volume is also increased when the erythrocyte distribution width is increased, the anemia is large-cell non-homogeneous, such as megaloblastic anemia and myelodysplastic syndrome. is reduced, it is small cell non-homogeneous anemia, such as iron deficiency anemia. Therefore, the width of red blood cell distribution is mainly used for the morphological classification of anemia, and the higher the width, the more it indicates that the shape and size of the red blood cells in the sample vary, and the specific causes need to be further clarified with medical history and relevant laboratory tests for diagnosis and treatment.