A transfusion may be needed when a patient has lost a significant amount of blood, such as after major trauma or surgery, or when a specific blood component is too low or is not functioning adequately. Red blood cells may be used for significant blood loss or certain forms of anaemia, while platelets or plasma may be given when clotting or platelet problems increase the risk of bleeding. The decision depends on the patient's condition, symptoms, blood results and overall clinical situation.

Blood can be separated into different components so that patients receive what they specifically need. Red blood cells carry oxygen to tissues, while platelets help blood clot and control bleeding. Plasma is the liquid portion of blood and contains proteins involved in clotting and other functions. Cryoprecipitate, which contains specific clotting proteins, may also be used in selected situations. Whole blood may be used in certain emergencies involving major blood loss, depending on clinical circumstances and local protocols.

Before a transfusion, the patient's blood is tested to determine the ABO blood group and Rh factor. The donated blood is then selected to be compatible with the recipient. For red blood cell transfusions, a process called crossmatching involves testing a sample of the patient's blood against the donor blood to check that they are compatible. Additional testing may be needed when a patient has previously developed certain red blood cell antibodies or has a history of transfusion reactions.

Donated blood undergoes careful screening and testing before it is made available for transfusion. Donor health information is assessed, and samples are tested for blood group and for infections that could potentially be transmitted through transfusion. Blood is also processed into components and stored under controlled conditions. These measures, together with careful donor selection, handling and compatibility testing, make transfusion-transmitted infections extremely rare, although no medical procedure is completely without risk.

Patients should tell their healthcare team if they have previously received a transfusion, experienced a transfusion reaction or been told that they have a red blood cell antibody. Before and during the transfusion, vital signs such as temperature, heart rate and blood pressure are monitored. Patients should immediately report symptoms such as fever, chills, itching, difficulty breathing, chest or back pain, or swelling. Most transfusions are well tolerated, but prompt reporting of symptoms allows the team to respond quickly if a reaction occurs.

Blood donation is generally intended for people who are in good health and meet the blood bank's age, weight and health requirements. Potential donors are usually asked about their medical history, medicines, recent illnesses, travel and other factors that could affect donation safety. Eligibility criteria vary between countries and blood banks, so meeting a general requirement does not automatically mean someone can donate on a particular day. The donor screening process protects both the donor and the person receiving the blood.