Erythrocyte distribution width is usually used together with the mean volume of red blood cells for morphological classification of anemia. Erythrocyte distribution width is a parameter that reflects the specificity of peripheral blood erythrocyte volume and is important for the diagnosis of anemia. Normal adults have an erythrocyte volume distribution width <0.15 (<15%). A low erythrocyte distribution width indicates that the sample blood erythrocyte morphology is uniform in size and very neat. When the average erythrocyte volume is increased: if the erythrocyte distribution width is normal, it is large cell homogeneous anemia, such as partial aplastic anemia; if the erythrocyte distribution width is increased, it is large cell non-homogeneous anemia, such as megaloblastic anemia and myelodysplastic syndrome. When the average volume of erythrocytes is normal: if the width of erythrocyte distribution is normal, it is normal cell homogeneous anemia, such as acute hemorrhagic anemia; if the width of erythrocyte distribution is increased, it is normal cell non-homogeneous anemia, such as aplastic anemia and paroxysmal sleep hemoglobinuria. When the average volume of erythrocytes is reduced: if the distribution width of erythrocytes is normal, it is small cell homogeneous anemia, such as dyscrasia, spherocytosis, etc.; if the distribution width of erythrocytes is increased, it is small cell non-homogeneous anemia, such as iron deficiency anemia. Therefore, the erythrocyte distribution width is mainly used for morphological classification of anemia. A low erythrocyte distribution width indicates that the sample blood has uniform erythrocyte morphological size.