Consideration of ABO Compatibility in Allocation of Hearts for Infant Transplantation

Pediatricians have long argued that children are not little adults, and it seems intuitive that clinical practices and regulations should follow appropriately from this paradigm. One can find no better evidence to support this contention than the report by Almond et al1 published in this issue of Circulation. ABO incompatibility has been considered a relative or, by United Network for Organ Sharing standards, an absolute contraindication to transplantation of hearts2 and, until recently and only under certain circumstances, kidneys.3 This practice is based on abundant evidence that ABOincompatible cardiac and renal grafts in unmodified recipients are often rejected very quickly. Early rejection of ABO-incompatible transplants is thought to be caused by antibodies direct against A or B antigens expressed on graft endothelium. These antibodies can initiate a cascade of events culminating in widespread graft thrombosis. Thus, the clinical protocols that evolved and gained eventual widespread acceptance initially for successful kidney transplantation stipulated that donors must be ABO compatible with any proposed recipient. Over time, further evidence suggested that ABO-identical transplantation resulted in superior results compared with ABO-compatible transplantation,4 presumably because of antibodies to minor blood group antigens. In the United States, blood group O recipients were found to be significantly disadvantaged in terms of obtaining organs by the practice of allowing unrestricted allocation to ABOcompatible nonidentical recipients (such as a group O donor to a group A recipient). Thus, on the recommendation of the Thoracic Organ Transplantation Committee, United Network for Organ Sharing rules for allocation of donor hearts in the United States were changed in 1999 to mandate allocation preferentially to ABO-identical over ABO-compatible recipients, regardless of age.5 It is ironic that this broad application of a change in regulations prompted by the well-intentioned need to remove an allocation rule that disadvantaged one group of potential transplant recipients (blood group O adults) appears to have resulted needlessly in an important disadvantage for another group (infants).