Principle:

Patient serum is tested with donor cells before blood is given. The methods used must demonstrate ABO incompatibility and detect clinically significant unexpected antibodies.
Coomb's reagent (AHG serum) is added after incubation of the donor cell with the patient's serum. If cells are sensitized with IgG antibodies, AHG will attach to IgG coated cells causing agglutination.

Specimen:
• Serum collected in plain tube is required.(5ml blood)
• Donor red cells from an originally attached segment.
• Sample for pretransfusion testing can be tested for 3 days limit.
(If patient has not been transfused or pregnant).
• Patient’s blood sample and donor cells (Unit Tag) should be kept at
1-6˚C for 7 days after transfusion.


Method:
1. Perform ABO grouping and D typing on unit RBC`s and patient's RBCs.
2. Perform Serum grouping of the patient using A cell and B cell suspensions.
3. Anti D control should be done to ensure the RH is correct.

4. Compatibility testing (The Coomb`s Cross Match):

• Wash donor cells 3 times in saline, make 5% suspension.
• Auto control is done using patient’s cells added to patient’s serum.
• In 4 test tubes, add to each 2 drops of patient’s serum, 2 drops of 5% cell suspension.
• Incubate tube 1(RT), at room temperature, the remaining 3 tubes (37ºC, AHG &Albumin) together with the auto control are incubated at 37ºC for an hour.
• Tube (RT), (37ºC) are centrifuged for 1 minute at 1000rpm, examine for hemolysis. Resuspend the cells and examine for agglutination.
• Wash cells in Tube 3(AHG) with normal saline, add 2 drops Coomb`s reagent, centrifuge for 1 minute at 1000rpm, examine for hemolysis. Resuspend the cells and examine for agglutination.
• Add albumin to Tube 4 (Albumin) without disturbing the cells, incubate at 37˚C for 30 minutes. Examine for hemolysis, Resuspend the cells and examine for agglutination.


Interpretation:
Any reactivity at any phase (RT, 37˚C, AHG or Albumin) whether detected as agglutination or hemolysis is considered Incompatible and the unit could not be issued for transfusion.