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

· Principles

· Blood groups

· Compatibility testing

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

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.

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 for ABO and RhD
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 for ABO (i.e. double check)
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	· Donor’s RBC + potential recipient’s serum · Mostly computer matching based on group and antibody screen · Occasionally by in vitro transfusion · Ensures almost 100% safety (cf. Group and antibody screen only: 98% safety)