Genotyping for Weak and Partial D Rh Status to Optimize Usage of Limited Blood Supplies

Abstract Introduction The Rh antigen group, expressed on erythrocytes and encoded by highly polymorphic RHD and RHCE genes on chromosome 1 (1p36-p34), is notable in transfusion medicine due to its high polymorphism, strong immunogenicity and ability to cause hemolytic transfusion reactions in patients and hemolytic disease of the fetus and newborn (HDFN). The D antigen specifically contains multiple epitopes with variations that include decreased expression (weak D) or loss of certain epitopes (partial D) which expose patients to risk of alloimmunization depending on the subtype of the Rh(D) antigen. Hence, serologic weak D testing followed by molecular testing is not performed except for donors testing negative for Rh(D) antigen and newborns typing negative for Rh(D) and born to Rh(D) negative mothers. Given the current shortages of blood supplies, understanding patients’ genotypes in the context of weak or partial D-typing and risk of alloimmunization can be essential in the management of increasingly limited supplies of red blood cell (RBC) units. Objective To further characterize patients that initially type negative for Rh(D) but positive for weak D with molecular testing to investigate risk of alloimmunization, retype the Rh(D) and avoid unnecessary use of Rh(D) negative RBC units during critical shortages of RBC units. Methods We collected the results of ABO/Rh(D) typing for patients with negative Rh(D) testing but positive weak D test over a period of four weeks in December 2023 and had D-variant molecular testing performed by a reference laboratory. Initial serologic ABO/Rh(D) was performed in tube followed by weak D testing by indirect Coombs test in tube. Results The total number of patients who initially tested negative for Rh(D) but positive (weak to +4) for weak-D was three patients. Two patients showed a probable genotype of RHD*01W.1 (RHD* weak type 1, 809T>G (V270G)) homozygous which indicated the presence of weak D+ type 1. Patients with weak D+ type 1 can receive Rh(D) positive red cell units, and they are not at risk of alloimmunization and developing anti-D. The third patient showed a probable genotype RHD*DVI type 2 homozygous (lack of Rh(D) exons 4 through 6) and indicated the presence of partial D antigen. Patients with partial D antigen should receive RH(D) negative red cell units as they are at risk of alloimmunization and developing anti-D. Conclusions Although the vast majority of Transfusion Medicine services do not perform the serologic weak D test with reflex to molecular testing, characterizing patients who test negative with initial serologic Rh(D) testing with serologic and molecular D-tests may aid in retyping the Rh(D) of patients during critical shortages, particularly for patients with ABO blood group O.