In ABO grouping, the testing of the patient’s serum or plasma against, at minimum, A1 and B reagent RBCs (the process is also known as “serum grouping” or the slang term “back typing“). The interpretation of this part of the ABO test is usually simple: Reactions against the A1 cells only shows that the person has anti-A and is therefore blood group B, for example. The blood group identified by the reverse grouping should be the same as that identified by the “forward” (or “cell“) grouping; if not, an ABO discrepancy is present.