The red blood cell volume distribution width (RDW) is obtained by measurement with a hematocrit analyzer and is a parameter that reflects the heterogeneity of the peripheral blood red blood cell volume. What it expresses is the degree of homogeneity of red blood cell volume size. If the red blood cell volume size is homogeneous and consistent, the parameter is lower and within the reference range. If the erythrocyte volume size is inconsistent and varies widely, the parameter is higher than the reference value. The morphological characteristics of erythrocytes are different in anemia caused by different etiologies. RDW can be used for early diagnosis and efficacy observation of iron deficiency anemia; RDW/MCV combination is also used for morphological classification of anemia, etc. The diagnosis is based on the following: 1. High erythrocyte pressure product: The erythrocyte pressure product helps to understand the increase and decrease of erythrocytes, and when the absolute value of erythrocytes increases due to various causes, there will be a corresponding increase in the erythrocyte pressure product. High erythrocyte pressure product is found in the patient’s blood tests after shock. 2, erythrocytosis – hyperviscosity: erythrocytosis – hyperviscosity syndrome is a relatively common disease in the neonatal period, due to erythrocytosis, blood stagnation, slow blood flow, prone to poor tissue perfusion and hypoxia and damage to the heart, brain, kidneys and other multiple organs, part can affect the long-term prognosis of the nervous system. 3, erythrocytosis: erythrocytosis refers to the number of red blood cells per unit volume of blood is higher than the high limit of the normal value. Relative increase is seen in hemoconcentration; absolute increase is seen in plateau life, fetus and newborn, strenuous physical labor, severe cardiopulmonary disorders and true erythrocytosis.