It's time to phase out “serologic weak D phenotype” and resolve D types with RHD genotyping including weak D type 4

I n 2015, the AABB and the College of American Pathologists sponsored an Interorganizational Work Group on RHD Genotyping, inviting the American College of Obstetricians and Gynecologists (ACOG), American Red Cross, Americaʼs Blood Centers, and Armed Services Blood Program to recommend organizational representatives. In addition, five scientific consultants were invited to be members of the Work Group. The Work Group developed and published recommendations for phasing-in RHD genotypings for individuals with a “serologic weak D phenotype,” focusing on pregnant women and potential recipients of a blood transfusion. During the subsequent 5 years, molecular immunohematology laboratories have observed an increase in the number of requests for RHD genotyping reflecting expanded implementation of RHD genotyping in clinical practice. The potential benefit if the Work Groupʼs recommendations were fully implemented, rather than treating all individuals with serologic weak D phenotype as D−, would be the elimination of an estimated 24,700 unnecessary anteand postpartum injections of Rh immune globulin (RhIG) annually in the United States. Also, as many as 47,700 units of D− red blood cells (RBCs), always in short supply, would be made available for patients with definite need for such units. As members of the previously convened Work Group, we have observed several impediments to the integration of RHD genotyping in clinical practice, which are the subject of this Commentary. The Work Groupʼs recommendation could not require the hospitals to alter their D antigen typing practice, and did not specify whether the interpretation of RHD genotyping results should be included in the laboratory report or whether laboratories should report only RHD genotyping results, deferring the interpretation to clinical practitioners. We have observed that when the molecular report does not provide an interpretation and recommendation, clinical practictioners not experienced in blood group genetics hesitate to interpret RHD genotyping results as D+ or D−. For the purposes of RhIG adminstration and transfusion, interpretation of an RHD genotype requires not only pertinent experience but also knowledge of the history as well as the current evolving data and literature reports of the risks for alloimmunization associated with specific RHD genotypes. Hospital computer systems may require updating to accomodate new requirements, such as the ability to change a prior serologic D typing result, when it is overridden by a different D type based on RHD genotyping results. We provide guidance on the interpretation and update our previous recommendation to include the management