RDW-SD is the coefficient of variation of red blood cell distribution width in peripheral blood, which is an indicator of the dispersion of red blood cell size in peripheral blood, that is, an indicator of whether the size of red blood cells in peripheral blood is homogeneous. If the RDW-SD is low, it means that the size of the red blood cells in peripheral blood is not very different, and the size is more homogeneous, and they may all be larger red blood cells, or they may all be smaller red blood cells. appears to be low. The second disease is megaloblastic anemia, which also has a low RDW-SD because of the homogeneous increase in the volume of red blood cells.