RBC distribution width SD, which is the width of red blood cell volume distribution, is an indicator of the dispersion of red blood cell volume size in peripheral blood, which is also an indicator of whether the size of red blood cells in peripheral blood is homogeneous. If the RBC distribution width SD is low, it means that the size of the red blood cells in peripheral blood is uniform and the size difference is not large. In clinical practice, the most common cause of low RBC distribution width is small cell hypochromic anemia caused by iron deficiency anemia and thalassemia, especially in thalassemia, where the red blood cells cannot synthesize normal hemoglobin due to impaired synthesis of peptide chains, resulting in a uniform decrease in the volume of red blood cells, which results in low RBC distribution width SD. Patients with iron deficiency anemia also have low RBC distribution width SD, but after iron supplementation treatment, the red blood cell volume starts to increase, and the RBC distribution width SD will gradually increase.