Hemoglobin is the protein that binds to and carries oxygen in the blood. A low hemoglobin is a condition called anemia Anemia can have several causes from blood loss, nutritional deficiencies, and metabolic conditions.
The practice of transfusing because of a certain number has been abandoned years ago. Prior to the 1980s, doctors commonly transfused when the hemoglobin decreased below 10. Research has shown no benefits to the patient when just transfusing to keep the number greater than 10. Patients should be transfused when they are symptomatic. Symptoms can include: chest pain, shortness of breath, rapid heart rate, or drops in their blood pressure with certain positions leading to dizziness. If the patient does not have these symptoms, oral iron tablets should be adequate.
The question you do need to concern yourself with is WHY is the hemoglobin decreased? Has the patient had recent surgery? Do they have blood in their stool? Are they a patient on kidney dialysis? Is their diet balanced?