Erythrocyte distribution width is a parameter that reflects the heterogeneity of red blood cell volume obtained by instrumental measurement, and is an objective indicator of the unequal size of red blood cells. A low erythrocyte distribution width alone indicates that the morphology of erythrocytes in the patient’s blood is similar in size and relatively neat, which has no special clinical significance and is generally not a cause for excessive concern. If there are other indicators abnormal at the same time of low erythrocyte distribution width, it may be iron deficiency anemia or megaloblastic anemia: 1. Iron deficiency anemia: during the occurrence of iron deficiency anemia, the size of erythrocytes will change, and the volume can be reduced to 60 fL or even smaller, i.e., small erythrocytes, manifesting as small cell hypochromic anemia, and patients with low erythrocyte distribution width may have erythrocytes The patient may have a low erythrocyte distribution width and a decrease in hemoglobin; 2. megaloblastic anemia: during the occurrence of megaloblastic anemia, the size of the red blood cells will change, and the volume may expand to 150 fL, i.e., large red blood cells, manifesting as megaloblastic anemia, and the patient may have a decrease in hemoglobin along with a low erythrocyte distribution width. Erythrocyte distribution width is a widely used test in clinical practice and is one of the routine blood tests. This test has the advantages of being simple, inexpensive, easy to obtain, and widely available. Erythrocyte distribution width is often used in the differential diagnosis of anemia and is closely related to cardiovascular diseases, kidney diseases and liver diseases.