Decreased hemoglobin is a decrease in the concentration of hemoglobin in peripheral blood, which is commonly seen in various cases of anemia. Common causes are as follows: i. Hemoglobin reduction can occur in acute blood loss, such as chronic hemorrhoid bleeding, chronic ulcer bleeding and bleeding from peptic tract malignant tumors, bleeding caused by excessive menstruation in women and ruptured bleeding from ectopic pregnancy, etc. Due to the loss of red blood cells, it can cause a decrease in hemoglobin concentration. Second, hemoglobin reduction caused by bone marrow hematopoietic disorders, such as aplastic anemia, acute leukemia, bone marrow metastatic cancer, myelodysplastic syndrome, paroxysmal sleep hemoglobinuria, etc., can cause a significant reduction in hemoglobin. Third, red blood cell lysis destruction causes hemoglobin reduction, and the most common disease is autoimmune hemolytic anemia.