2000B04 Outline the principles of compatibility testing of allogeneic (homologous) blood for transfusions.

 

List:

·         Principles

·         Blood groups

·         Compatibility testing

 

Principles:

Grouping

·   RBCs have membrane protein antigens (Ag)

·   Antibodies (Ab) are produced against non-self RBC antigen after exposed

·   Aim of grouping is to determine ABO and RhD antigen presence on RBC

Transfusion

·   Allogeneic transfusion only safe if recipient won’t make antibodies against donor’s antigens

·   Donor plasma removed from packed RBC hence donor antibodies rarely matter

·   O- is universal RBC donor, AB+ universal recipient

·   AB+ is universal plasma donor, O- universal recipient

Compatibility requirements

·   Red cells: ABO, +/-RhD, other antibodies

·   Platelets: ABO, +/-RhD preferred

·   Plasma products: ABO

Special considerations

·   Test valid for a few days in hospital because:

o Existing titres may change

o New antibodies may be formed

·   Some antibody-antigen interactions more important than others

·   RhD compatibility required if RhD- female patient of child-bearing potential

 

Blood groups:

ABO

·   Most important system

·   A: A antigen

·   B: B antigen

·   O: neither antigen

·   Incompatibility -> intravascular haemolysis

·   Antigen: is carbohydrate, more antigenic

·   Antibodies: developed soon after birth due to diet exposure; more IgM, does not cross placenta

RhD

·   Next most important system

·   Incompatibility -> extravascular haemolysis

·   Antigen: is protein, less antigenic

·   Antibodies: developed only upon direct blood exposure; more IgG, does cross placenta

Other

·   Kell, Kidd, Duffy, Lewis etc.

 

Compatibility testing:

Forward typing

·   Patient’s RBCs + serum solutions known to have anti-A, anti-B or anti-RhD

o ABO: observe for agglutination (since IgM, pentameric)

o RhD: wash away serum, add Coombs reagent, observe for agglutination (since IgG, monomeric)

·   Performed every time

Reverse typing

·   Patient’s serum + RBC solutions known to have A Ag, B Ag or RhD A

o ABO: as above

o RhD: as above

·   Performed sometimes for confirmation

Antibody screen

·   Patient’s serum + RBC solutions known to have various RBC antigens (e.g. Kell, Kidd, Duffy)

·   Wash away the serum

·   Add Coombs reagent, observe for agglutination (since IgG, monomeric)

Cross-match

·   Mostly computer matching based on group and antibody screen

·   Occasionally by in vitro transfusion: Donor’s RBC + potential recipient’s serum

·   Observe for agglutination

·   Almost 100% safe

·   (Group and antibody screen only: 98% safe)

 

 

 

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