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Yes, of course, which is why you MUST use enzyme-treated group O red cells (screening cells - not just one example of group O red cells) as a negative control. If you do, and the group O red cells give a reaction, you still don't know the ABO group, until you have sorted out the specificity, and then used reverse grouping cells - and group O - cells negative for the cognate antigen. If you don't use a negative control - you should be sacked!!!!!!!!!! Nothing personal!