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ABO blood grouping is a key diagnostic point, which is essential to ensure security of blood transfusions. This analysis must comply with the rules currently in force in our national legislation. It also requires to carry out internal quality checking (IQC) every day. These IQC include the whole process and allow analytical validation of ABO blood grouping. The objective of IQC is to detect all the abnormal events generated by a defective reagent or technology. The importance of ABO blood grouping is underscored by the fact that all ABO blood groupings consist of both a cell grouping and a plasma grouping. In most cases, the interpretation of these two tests supports a common conclusion and the ABO group is confirmed. It is essential to be able to detect mixed-field but in some instances, cell grouping and plasma grouping result in different interpretations of the ABO type of the patient. In these cases, a cell-plasma group discrepancy exists. Common causes of discrepancies are listed in the paper.