Blood samples compatibility testing
Any failure to follow correct procedures can lead to incompatible transfusions. Blood bank staff are acting correctly if they refuse to accept a request for compatibility testing when either the blood request form or the patient’s blood sample are inadequately identified or the details do not match. If there is any discrepancy, they should request a new sample and request form.
It is essential that all blood is tested before transfusion in order to:
• Ensure that transfused red cells are compatible with antibodies in the recipient’s plasma
• Avoid stimulating the production of new red cell antibodies in the recipient, particularly anti-RhD. All pre-transfusion test procedures should provide the following information about both the units of blood and the patient:
• ABO group
• RhD type
• Presence of red cell antibodies that could cause haemolysis in the recipient.
ABO blood group antigens and antibodies
The ABO blood groups are the most important in clinical transfusion practice. There are four main red cell types: O, A, B and AB. All healthy normal adults of group A, group B and group O have antibodies in their plasma against the red cell types (antigens) that they have not inherited:
• Group A individuals have antibody to group B
• Group B individuals have antibody to group A
• Group O individuals have antibody to group A and group B
• Group AB individuals do not have antibody to group A or B. These antibodies are usually of IgM and IgG class and are normally able to haemolyse (destroy) transfused red cells.
Anti-A or anti-B recipient antibodies are almost always capable of causing rapid destruction (haemolysis) of incompatible transfused red cells as soon as they enter the circulation. A red cell transfusion that is not tested for compatibility carries a high risk of causing an acute haemolytic reaction. Similarly, if blood is given to the wrong patient, it may be incompatible.
The exact risk depends on the mix of ABO groups in the population. Typically, at least one third of unmatched transfusions will be ABO incompatible and at least 10% of these will lead to severe or fatal reactions. In some circumstances, it is also important that the donor’s antibodies are compatible with the patient’s red cells. It is not always essential, however, to give blood of the same ABO group.
RhD red cell antigens and antibodies
Red cells have many other antigens but, in contrast to the ABO system, individuals very rarely make antibodies against these other antigens, unless they have been exposed to them (‘immunized’) by previous transfusion or during pregnancy and childbirth.
The most important is the RhD antigen. A single unit of RhD positive red cells transfused to an RhD negative person will usually provoke production of anti-RhD antibody. This can cause:
• Haemolytic disease of the newborn in a subsequent pregnancy
• Rapid destruction of a later transfusion of RhD positive red cells.
Other red cell antigens and antibodies
There are many other antigens on the human red cell, each of which can stimulate production of antibody if transfused into a susceptible recipient.
These antigen systems include:
• Rh system: Rh C, c, E, e
• Lewis. These antibodies can also cause severe reactions to transfusion.
Pre-transfusion testing (compatibility testing)
A direct test of compatibility (crossmatch) is usually performed before blood is infused. This detects a reaction between:
• Patient’s serum
• Donor red cells.
The laboratory performs:
• Patient’s ABO and RhD type
• Direct compatibility test or crossmatch. These procedures normally take about 1 hour to complete. Shortened procedures are possible, but may fail to detect some incompatibilities.
1. If the patient’s sample has a clinically significant red cell antibody, the laboratory may need more time and may require a further blood sample in order to select compatible blood. Non-urgent transfusions and surgery that is likely to require transfusion should be delayed until suitable blood is found.
2 If transfusion is needed urgently, the blood bank and the doctor responsible for the patient must balance the risk of delaying for full compatibility testing against the risk of transfusing blood that may not be completely compatible.