Erythrocyte distribution width SD refers to an index that reflects the size and dispersion of red blood cells, that is, whether the size of red blood cells in peripheral blood is uniform. A wide distribution width of erythrocyte fractionation indicates that the size of erythrocytes is not homogeneous, with large ones being large and small ones being small. If the erythrocyte distribution width is reduced, it indicates that the erythrocytes in the peripheral blood are uniform in size, with little disparity in size. Erythrocyte distribution width is commonly seen in various anemic diseases in clinical practice, for example, patients with iron deficiency anemia start with small cell hypochromic anemia and have narrow red blood cell distribution width. However, if iron supplementation therapy is given, the volume of red blood cells starts to increase gradually, and then the width of red blood cell distribution increases significantly, because some red blood cells increase in volume and some do not yet. When the hemoglobin concentration is normal and the red blood cell volume returns to the normal range after the iron supplementation therapy is finished, the red blood cell distribution width becomes smaller again and returns to the normal range.