Erythrocyte distribution width, also known as erythrocyte volume distribution width, is an indicator of the size and dispersion of erythrocytes in the peripheral blood, that is, whether the size of erythrocytes in the peripheral blood is homogeneous. A low erythrocyte distribution width indicates that the size of erythrocytes in peripheral blood varies little. There are two most common reasons for this: one is that the volume of erythrocytes in peripheral blood is all small, and the most common diseases include thalassemia, iron deficiency anemia, etc. The volume of red blood cells in the peripheral blood is small for various reasons, which results in a significantly lower width of red blood cell distribution. For example, patients with iron deficiency anemia have microcytic hypochromic anemia, but when iron supplementation is given, the volume of red blood cells starts to increase, and the low width of red blood cell volume distribution improves. Another type of disease in which the volume of red blood cells in the peripheral blood is large is megaloblastic anemia. As the volume of all the red blood cells in the peripheral blood is significantly larger, the condition of low width of red blood cell volume distribution also occurs. When treated with folic acid and vitamin B12 supplementation, the low erythrocyte volume distribution width also improves significantly.