Hemoglobin is a special protein that transports oxygen within the red blood cells and is one of the most important indicators in routine blood tests. In clinical practice, the degree of anemia is generally defined by the concentration of hemoglobin. The World Health Organization suggests that hemoglobin <130g/L in adult males and <120g/L in adult females can be diagnosed as anemia. Therefore, anemia can be diagnosed if routine blood tests show hemoglobin below these standards. There are many causes of anemia, as follows: 1) decreased synthesis of red blood cells: such as aplastic anemia, iron deficiency anemia, megaloblastic anemia, etc.; 2) increased destruction of red blood cells: such as hemolytic anemia; 3) excessive loss of red blood cells: such as heavy bleeding and excessive menstrual flow in women. Regardless of the cause of anemia, patients need to go to the hospital as soon as possible to identify the cause and treat accordingly so as not to delay the condition.