Effect of blood group active micro-organisms on the ABO grouping of human whole saliva

Three saliva samples with false positive ABO grouping results were assayed for blood group active organisms, using a variety of selective media to isolate representative strains from the salivary microflora. Eight out of 40, 8 out of 40 and 4 out of 30 strains from the three samples, respectively, showed blood group activity, which correlated well with the false positive specificities of the saliva samples. In all cases the false reaction only lasted a few days. Investigation of one of these samples before and after the appearance of the false positive activity yielded only one out of 40 blood group active organisms, using the same methods. Similar investigation of two "normal" saliva samples found none out of 40 and one out of 40 blood group active organisms, respectively. It is concluded that occasional false positive ABO grouping reactions of saliva samples are probably caused by the presence of unusually high numbers of blood group active micro-organisms, due to disturbances in the ecological balance of the salivary microflora.