The Essentials of Pre-Transfusion Testing

The blood bank has a vital role in the running of any hospital. Blood is essential to a whole range of clinical scenarios. Think of patients with major trauma, or post-partum haemorrhage, and chronic anaemia conditions like some thalassaemia syndromes. Without the availability of blood products, clinical management would be very difficult, if not impossible.

Getting the right blood to the right patient is a central principle in providing safe and effective blood. Pre-transfusion testing in the provision of red cells is important to avoid potentially fatal complications to the patient. With only a few exemptions, an ABO incompatible transfusion will result in very high risk of death. In one study, patients who received more than 50 mL of incompatible blood, 64% manifested signs or symptoms related to the incompatible transfusion, and 17% died. Such is the seriousness of this error, it is classed as a never event. 

Delayed haemolytic transfusion reactions although may not have the same instant and harmful reaction, can also cause significant morbidity. What is the main reason for these problems? Well, this stems from incompatibility between the patient and donor red cell antibodies and antigens.

Red cell antibodies

There are two main types of red cell antibodies

1. IgG antibodies

• react at body temperature (37oC)
• coat the red cell but do not directly lyse red cells
• are able to cross link the red cells together and cause agglutination
• they are induced by pregnancy and transfusion
• can cross the placenta and potentially induce haemolytic disease of the fetus and newborn
• examples include anti-RhD, anti-Rhc, anti-Rhe, anti-Duffy

2. IgM antibodies

• reacts best below body temperature (37oC)
• are naturally occurring
• cannot cross the placenta
• are directed against the ABO blood group system where they bind and fix the complement system causing rapid red cell lysis
• others are generally not clinically significant, for example, anti-Lewis and anti-P1PK

More about ABO compatibility testing – the forward and reverse methods

ABO compatibility testing is therefore one of the key steps in ensuring the patient is provided with safe blood. 

There is a reciprocal relationship between ABO antibodies and antigens. For example, someone with group A antigen on their red cells will have anti-B antibodies in their plasma. Whilst an absence of any ABO red cell antigens means they have both anti-A and anti-B antibodies in their plasma.

ABO testing can be performed using two principles

1. Forward grouping: red blood cells are tested against known anti-A and anti-B sera.
2. Reverse grouping: serum is tested against known A and B red cells.

A positive reaction results in red cell agglutination (the red cells clump together).

The forward and reverse reactions should match e.g. group O blood no reaction in the forward group but reaction in the reverse group.

The table below shows the expected reactions seen in the four ABO blood groups in both the forward and reverse reactions.

+ agglutination
– absent agglutination