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Seventy-one ABO incompatible (heterospecific) infants and 71 controls, who were free from other potential causes of jaundice, were studied to ascertain which cord blood tests reliably predict the severity of ABO haemolytic disease of the newborn (ABO HDN). The modified Direct Antiglobulin Test (spin DAGT) was positive in all infants who required treatment for haemolytic jaundice and only DAGT positive children showed evidence of impending haemolytic anaemia or compensated haemolysis in cord or capillary blood. Cord serum bilirubin concentration had some predictive value, particularly when the level exceeded 85 mumol/l, but it was a less reliable indicator and had greater value if used in association with the DAGT. The elution test, which is frequently used as a diagnostic tool in ABO HDN, had no predictive value and we felt that its putative value is due to overdiagnosis of ABO HDN in jaundiced heterospecific infants. We conclude that the spin DAGT, despite the weakness of the reaction, reliably identifies infants at risk from severe ABO HDN and is sufficiently sensitive to be used as a single screening test for the early detection of the disorder.

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A new genetic model of the P blood group system is presented. The system is controlled by two chromosomal loci. The first locus has three allelic genes. The pk gene codes for an alpha galactosyl transferase that converts ceramide dihexoside to ceramide trihexoside (or the pk antigen). The second allele, the pk gene, codes for an alpha galactosyl transferase that converts both ceramide dihexoside to ceramide trihexoside (or the pk antigen) and paragloboside to the P1 antigen. The third allele does not produce an active codes for a beta N-acetyl galactosaminyl transferase that converts ceramide trihexoside to globoside (or the P antigen). The second allele does not produce an active product. The predictions of the model are in agreement with family studies and fibroblast fusion studies. The current model and previous genetic models, however, predict different possible phenotypes from rare 2 x p or p2k x p matings or fibroblast fusions.