Erythrocyte volume distribution width (EVRW) is a test used in clinical practice for the classification, differential diagnosis and early diagnosis of anemia. A high erythrocyte volume distribution width is often a precursor of iron deficiency anemia and hemolytic anemia. The erythrocyte volume distribution width is an indicator of the dispersion of erythrocyte size in peripheral blood, and a high erythrocyte volume distribution width indicates a large difference in the size of erythrocytes in peripheral blood. In iron deficiency anemia, iron deficiency causes small cell hypochromic anemia, and some of the red blood cells are significantly reduced in size, resulting in a larger volume difference between red blood cells. In the same way, in hemolytic anemia, the width of the red blood cell volume distribution is high due to the large size of some red blood cells.