Red blood cell distribution width, abbreviated RDW, is a parameter that reflects the heterogeneity of red blood cell volume in peripheral blood, that is, an indicator of whether red blood cells are normal in size, and is measured by a blood cell analyzer. A normal red blood cell volume distribution width indicates that the red blood cells in the blood are of uniform size and do not differ greatly from one another. If the red blood cell distribution width increases, it means that the red blood cells in the blood are not uniform in size and vary greatly. Increased width of red blood cell distribution is commonly seen when iron deficiency anemia and megaloblastic anemia are effectively treated. Because the red blood cells in the blood are small before the treatment of iron deficiency anemia, after the treatment of iron supplementation, the size of the red blood cells gradually increases, and the width of the red blood cell distribution increases. Before the treatment of megaloblastic anemia, the red blood cells are huge in size, but after the treatment, the volume of red blood cells gradually decreases, and the width of red blood cell distribution also increases.