Well, the answer is yes and no! Yes, we do the former (test at a dilution of 1/100 against both A and B red cells on an Olympus), but if there is a reaction we do not mark the units as high titre positive! Rather, we marked those that do NOT react as high titre negative (HT-). For a reason unbeknown to me (I think it has something to do with EU Regulations, but I'm not sure), and apart from ABO and the D, C, c, E and e antigens of the Rh Blood Group System, anything else for which we test is only printed onto the unit label if the test is negative. For example, if we test for HbS, and we find the donor to be HbS-, then HbS- is printed on the unit label, but if we find the donor to be HbS+, we do not print HbS+ on the label - the HbS status is just not shown. The same applies for blood groups. If we test with anti-K, and we find the donor to be K-, then K- is printed on the unit label, but if we find the donor to be K+, we do not print K+ on the label - the K status is just not shown. As I say, why I just do not know. :(