Sorry that I came to late to this topic. You are right, It is for financial reason and time consuming that it was decided to confirm an group O unit using aanti-A,B. As of the weak subgroups of A or AB that front type negative with anti-A,B., it is very true that in this case when you look at the all reaction you will have a discrepancy that will prompt you to a weak or absence antigen in front reaction. Thanks