The page below is a sample from the LabCE course ABO Typing Discrepancies. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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ABO Testing and Interpretation

Prior to patient testing, all reagents (anti-sera and cellular products) must be quality controlled to verify accuracy and acceptability. The reagents should also be inspected daily to ensure that they are in-date and do not appear to be contaminated.
The chart below lists expected results for quality control performance in a typical blood banking laboratory.

Cellular Reagents
Anti-A Anti-B Anti-A,B
A1 cells
2+ to 4+
0 (negative)
2+ to 4+
B cells
2+ to 4+
2+ to 4+
Although anti-A,B is not typically used in patient testing, it is important to maintain quality control for this reagent as it is used in donor unit ABO confirmation testing.
Once quality control performance is complete and found to be acceptable, patient testing may proceed. Charts below provide examples of A, B, AB, and O testing reactions demonstrated with both forward and reverse typing

Anti-AAnti-BA1 cells
B cells
A 4+ 0
0 4+
B 0 4+ 4+ 0
AB 4+ 4+ 0 0
O 0 0 4+
Most blood banks or transfusion services require that reactivity for anti-A and anti-B testing reagents demonstrate a 2+ reaction or greater in order to definitively and safely type the blood group. This course module will address typing discrepancies that demonstrate weaker reactions.