Blood transfusion

A blood transfusion is a medical procedure in which blood or blood components are transferred intravenously into a patient’s circulatory system. Transfusions are used to restore vital components of the blood and to treat a wide range of clinical conditions, including anaemia, blood loss from trauma or surgery, clotting disorders, and certain haematological diseases. While historically transfusions used whole blood, modern practice typically relies on specific components such as red blood cells, plasma, platelets, and clotting factors, each administered according to clinical need.

Medical Uses

Red cell transfusionRed blood cells (RBCs) are transfused to improve oxygen delivery to tissues. Earlier clinical practice relied on transfusion when haemoglobin levels fell below 100 g/L, but evidence-based guidelines now recommend a more restrictive strategy. For most stable hospitalised patients, transfusion is considered when haemoglobin falls between 70 and 80 g/L, with decisions guided by symptoms, comorbidities, and oxygenation status. A single-unit transfusion, followed by reassessment, is standard for non-bleeding patients. Exceptions include those with active cardiac ischaemia or severe respiratory compromise, in whom higher thresholds may apply.
When anaemia is due to iron deficiency, and the patient is stable, iron supplementation—oral or parenteral—is preferred over transfusion. Other components, such as fresh frozen plasma, cryoprecipitate, and platelets, are used to treat coagulation deficiencies or thrombocytopenia.

Procedure

Ensuring safe transfusion practice requires multiple steps:
Blood donationMost transfused blood comes from voluntary donors. Donations are collected as whole blood or via apheresis, which selectively gathers red cells, platelets, or plasma. Donations are traceable throughout the entire chain from collection to administration, allowing investigation of any transfusion-related events.
In high-income countries, voluntary non-remunerated donation predominates; in many low-income settings, replacement or paid donations remain common. Motivations for donating blood often include altruism, while deterrents may include fear or distrust.
Processing and testingCollected blood is separated into its components through centrifugation. Plasma may be further processed into products such as albumin, immunoglobulins, clotting factors, or fibrinogen concentrate.
To ensure safety, all donated blood is tested for transfusion-transmissible infections, including:

• HIV
• Hepatitis B
• Hepatitis C
• Syphilis

Where relevant, it may also be tested for pathogens such as Trypanosoma cruzi (Chagas disease) or Plasmodium species (malaria). Despite global improvements, some countries still lack routine access to all required test kits.
All donations undergo ABO and RhD blood group testing. Platelet products may additionally be screened for bacterial contamination because they must be stored at room temperature.
Donors may be tested for cytomegalovirus (CMV) in contexts where CMV-negative products are needed, such as for certain immunocompromised recipients. However, leukoreduction, the removal of white blood cells by filtration, significantly reduces the risk of CMV transmission and lowers the incidence of febrile reactions and alloimmunisation.
Pathogen reduction technologies, such as treatment with riboflavin and ultraviolet light, are increasingly used to inactivate viruses, bacteria, parasites, and donor white cells. These methods can serve as alternatives to irradiation for preventing transfusion-associated graft-versus-host disease.

Compatibility Testing

Compatibility testing—performed before any transfusion—is essential to prevent adverse reactions. The process includes:

• Type and screen to determine the recipient’s ABO and Rh type and to detect unexpected antibodies.
• Antibody investigation, if the screen is positive, using a panel of phenotyped donor cells to identify clinically significant alloantibodies.
• Crossmatching, which involves mixing recipient serum with donor red cells to ensure compatibility. Immediate-spin or electronic crossmatching may be used when no significant antibodies are present.

Patients with clinically significant alloantibodies must receive red cells lacking the corresponding antigen. If an antibody is suspected, donor units are phenotyped and antigen-negative units selected.

Safety and Monitoring

During transfusion, patients must be monitored closely for signs of transfusion reactions, which range from mild febrile responses to severe complications such as acute haemolytic transfusion reactions or transfusion-related acute lung injury. Post-transfusion monitoring ensures early detection and management of adverse events.
Blood transfusion remains a cornerstone of modern medicine. Through improvements in donor screening, processing, compatibility testing, and pathogen reduction, the procedure has become increasingly safe and effective, supporting patients in emergency care, surgery, oncology, haematology, and chronic disease management worldwide.