Erythrocyte volume distribution width is an indicator of the dispersion of red blood cell volume size in peripheral blood, which is an indicator of the uniformity of red blood cell size in peripheral blood. A high standard deviation of erythrocyte volume distribution width indicates that the size of erythrocytes in peripheral blood is not uniform or disparate. A high standard deviation of erythrocyte volume distribution width can be seen in a variety of clinical conditions, including iron deficiency anemia. Due to iron deficiency, red blood cells are unable to use iron to synthesize normal hemoglobin, which can lead to a situation where some red blood cells are small in size, and the small red blood cells appear to have a large difference in size from normal red blood cells. After iron supplementation treatment, the small red blood cells gradually increase in size and the high standard deviation of red blood cell distribution width improves significantly. In addition, patients with megaloblastic anemia also have a significant increase in the standard deviation of erythrocyte volume distribution width due to the increase in the volume of some erythrocytes. When folic acid and vitamin B12 supplements are given, the high standard deviation of erythrocyte volume distribution width also improves significantly.