Guidelines for blood transfusion have been issued by many health institutions or organizations in different countries. According to these guidelines, red blood cells (RBCs) transfusion should be given when the hemoglobin level is less than 6g/dL or 7g/dL and is unnecessary when the level is more than 10g/dL. However, in all the guidelines, the determination of whether RBCs should be administered when the hemoglobin level is in the range of 7~10g/dL is based on the judgment from anesthesiologists or surgeons on the patient's condition including intravascular volume status, ongoing bleeding, any risk factors for vital organs ischemia or hypoxia, and so on. Index of transfusion trigger for patients with hemoglobin level between 7/dL and 10g/dL is necessary and important in clinical practice.