Red blood cell volume distribution width (RDW) is a parameter that reflects the specificity of peripheral blood red blood cell volume and has important reference significance for the diagnosis of anemia. The normal adult erythrocyte volume distribution width is <0.15 (<15%). A low erythrocyte volume distribution width indicates that the sample blood erythrocyte morphology is uniform in size and very neat. However, erythrocyte volume distribution width cannot be interpreted independently and is usually used together with mean erythrocyte volume for morphologic classification of anemia, but RDW analysis is for reference only and cannot be interpreted independently or mechanically. When the mean erythrocyte volume is increased: If the mean erythrocyte volume distribution width is normal, the anemia is large cell homogeneous, such as partial aplastic anemia; if the mean erythrocyte volume distribution width is increased, the anemia is large cell non-homogeneous, such as megaloblastic anemia and myelodysplastic syndrome. When the average red blood cell volume is normal: If the width of red blood cell volume distribution is normal, it is normal cell homogeneous anemia, such as acute hemorrhagic anemia; if the width of red blood cell volume distribution is increased, it is normal cell non-homogeneous anemia, such as paroxysmal sleep hemoglobinuria. When the average red blood cell volume is reduced: if the red blood cell volume distribution width is normal, it is small cell homogeneous anemia, such as pearl protein production disorder anemia; if the red blood cell volume distribution width is increased, it is small cell non-homogeneous anemia, such as iron deficiency anemia. Therefore, a low erythrocyte volume distribution width only indicates a consistent morphological size of erythrocytes in the sample blood and cannot be interpreted independently and mechanically. The specific cause needs to be further clarified with medical history and relevant laboratory tests for diagnosis and treatment of the specific cause.