Blood transfusion may be required in the perioperative period for patients who have lost or actively losing blood. In order to manage this scenario, it is essential you know the patient’s circulating blood volume (based on their weight and age) and be able to estimate how much blood has been lost, which is not always straightforward, as some blood loss may be concealed. A blood loss of less than 10% of total blood volume does not usually require a transfusion; blood loss of greater 30% of total blood volume invariably requires transfusion.

Laboratory and/or point of care testing can be invaluable to guide to both blood transfusion and other blood components such as FFP platelets. Every hospital will have a major haemorrhage protocol.

There are many complications associated with blood transfusion including fever, hypothermia, circulatory overload and lung injury, immunological reactions, acid–base disturbances, hyperkalaemia and hypocalcaemia. In addition, infections may be transmitted such as HIV and hepatitis. Finally a serious error is to transfuse the wrong blood to a patient caused by errors including blood bottle mislabelling, or not checking the blood against the patient’s wristband.