Blood compatibility test

[Ref: WG21:p540; PK1:p252-253; KB2:p186-187]

Aim = To prevent haemolysis of the transfused RBCs

To do so, 3 steps:

1. Blood typing
2. Antibody screen
3. Crossmatch
   * Saline test to check ABO grouping
   * Indirect Coomb's test to detect haemolytic IgG antibodies

Basis of compatibility testing

When blood is transfused into an individual with an incompatible blood type...

After transfusion, the plasma in the transfusion is so diluted
--> Agglutinins in plasma rarely causes agglutination
* i.e. the antibodies in the transfused plasma does not cause damage to recipient's own RBC

When recipient's plasma has agglutinins against the donor's RBC
--> Donor's RBC agglutinate and hemolyse
--> Free Hb liberated into plasma
--> Transfusion reaction

Thus,

Group O blood (with no RBC antigen but both anti-A and anti-B antibodies) can be given to anyone
* But Group O individual can only receive Group O blood
* O = Universal donor

Group AB blood (with both A and B antigen, but no antibodies) cannot be given to anyone other than group AB
* But Group AB individual can receive any group type
* AB = Universal recipient

NB:

• Antibodies against red cell agglutinogen (e.g. A antigen)
  = Agglutinins (e.g. anti-A antibody)
• Agglutination occurs when RBCs are close enough to allow antibodies to link to adjacent cells

Methods of compatibility testing

In crossmatching, donor RBCs are mixed with recipient plasma on a slide and checked for agglutination

Methods available:

1. Saline agglutination test
2. Albumin technique (or the papain agglutination test)
3. Low ionic strength saline (LISS) test
4. Antiglobulin test

Saline agglutination test

Involves donor's RBC being suspended in saline and tested against antibodies at room temperature

Used for detected IgM antibodies
* e.g. anti-A, anti-B, anti-Lewis, anti-P, anti-M, and anti-N

Albumin or papain agglutination test

When RBC are suspended in solutions containing free ions
--> Electric repulsion (zeta potential) between cells occurs as a result of negatively charged surfaces

Repulsion is reduced by presence of albumin or enzyme treatment of RBC with papain
* Papain removes negatively charged carbohydrates (e.g. sialic acid) from the cell surface

Used to enhance agglutination
* Usually for IgG antibodies

Low ionic strength saline test

Suspended red cells are used to enhance the activity of some antibodies

Used to provide a rapid and sensitive method for detecting most blood group antibodies

Antiglobulin (Coomb's) test

Used to detect incomplete IgG and IgM antibodies
* e.g. Kidd and Duffy antibodies

 

Risk of reaction

[KB2:p187]

• ABO-compatible blood
  --> Relative safety = 99.4%
• ABO and Rh compatible blood
  --> Relative safety = 99.8%
• ABO and Rh compatible blood plus negative antibody screen
  --> Relative safety = 99.94%
• ABO and Rh compatible blood plus negative antibody screen and Coomb's test
  --> Relative safety = 99.95%