Anemia is a clinical syndrome that occurs when the volume of red blood cells in the peripheral blood is lower than normal, and is often expressed clinically by the hemoglobin concentration and the severity of anemia. According to the decrease of hemoglobin concentration, anemia is classified into four degrees: mild anemia with hemoglobin between 90-120g/L; moderate anemia with hemoglobin between 60-90g/L; severe anemia with hemoglobin between 30-60g/L; and very severe anemia with hemoglobin below 30g/L. There are various causes of anemia, the most common one clinically is iron deficiency anemia, which is caused by insufficient iron intake, impaired absorption or chronic blood loss. Next, aplastic anemia, megaloblastic anemia, thalassemia, hemolytic anemia, etc. can occur. Regardless of the type of anemia, it can be mild anemia, moderate anemia or severe anemia, and all of them need to be diagnosed and treated at the hospital in time.