Anemia is defined as a decrease in the total number of red blood cells in the systemic circulating blood to below normal values. However, since the technique of measuring the total amount of red blood cells in the systemic circulating blood is complicated, clinically it generally refers to the concentration of hemoglobin in peripheral blood being lower than the normal standard for the same age group, sex and region of the patient. The domestic normal standard is slightly lower than the foreign standard. In coastal and plain areas, hemoglobin can be considered anemic if it is lower than 12.5 g/dl in adult men and 11.0 g/dl in adult women. hemoglobin in children under 12 years old is about 15% lower than the normal value for adult men, and there is no significant difference between boys and girls. It is generally higher in areas with high altitude. The decrease in red blood cell count is generally proportional to the decrease in hemoglobin concentration, but the decrease in red blood cell count in microcytic hypochromic anemia is relatively less than the decrease in hemoglobin, so that the red blood cell count may not fall below normal in mild anemia. In contrast, in macrocytic anemia, the hemoglobin concentration is relatively high and the red blood cell count is low. When the total blood volume has not returned to normal after water loss, water retention, or acute massive blood loss, the hemoglobin concentration does not accurately reflect the true extent of anemia, so the effect of these factors on anemia should be considered clinically. In addition, during acute massive intravascular hemolysis, the plasma contains higher concentrations of free hemoglobin, and the results of hemoglobin measurement in this case are higher than the actual degree of anemia. In this particular case, the red blood cell pressure and red blood cell count better reflect the degree of anemia.