Identifying obstetrics patients in whom RHD genotyping can be used to assess risk of D alloimmunization

The D antigen is highly immunogenic and may cause allo-immunization to occur after blood transfusion or pregnancy. Some RHD variant alleles express a D antigen that is missing one or more epitopes, thus putting a presumed D+ patient at risk for alloanti-D and hemolytic disease of the fetus and newborn. It is generally accepted that individuals who have a serologic weak D phenotype due to one of three alleles common in Caucasians, RHD*weak D types 1, 2, or 3, are not at risk for alloimmunization. In this study, blood samples from 46 obstetrics patients from a local health system were identified based on discrepant results between automated gel and manual tube testing (n = 20) or based on presentation with a serologic weak D phenotype (n = 26). RHD genotyping was performed using commercial and laboratory-developed tests. Of the 26 serologic weak D samples, 18 (69.2%) were found to carry alleles RHD*weak D type 1, 2, or 3. The remaining eight samples (30.8%) were found to carry partial D alleles. Of the 20 samples submitted because of D typing discrepancy, 7 (35%) carried alleles RHD*weak D type 1, 2, or 3, while 13 (65%) carried partial RHD alleles. This report summarizes the findings of one hospital system and its approach to integrating RHD genotyping into its assessment of risk of alloimmunization in obstetrics patients. It demonstrates that individuals with partial RHD alleles can present with serologic weak D phenotype, such that, without RHD genotyping, these individuals may not be identified as candidates for Rh immune globulin. The study also demonstrates that use of two methods (automated gel and tube testing) allows for identification of partial D cases that would otherwise be missed.