RHD Genotyping of Blood Donors May Avoid Anti-D Immunization.
Authors: Flegel Willy A. , Gabriel Christian , Gassner Waltraud , Ruff Heike , Wagner Franz F. 
Published: October 2019

Abstract
Antigen D compatible transfusion is standard practice in modern transfusion therapy, warranting proper antigen D typing in all blood donors. Blood group genotyping is increasingly utilized for prenatal diagnosis and after recent transfusions. RHD genotyping is of practical importance to overcome the limitations of standard serology, which frequently fails to detect some weak D and chimeric red blood cell (RBC) populations, and to enhance clinical safety for blood transfusion recipients. Recently, the transfusions of blood units from chimeric donors were reported to having induced an acute transfusion reaction and multiple anti-D immunizations. The latter donor escaped the serologic detection in 13 donations but was uncovered by the strategy reported in this study. Since January 2002 all D negative (D neg.) first time donors at blood center A were tested for carrying the RHD gene. We evaluated the results of this two years’ routine testing and compared them to a screening study conducted in the independent blood center B. This approach contributed to define the utility of RHD genotyping for the quality control of D neg. RBC units.
In two independent blood centers we examined 9,931 and 5,115 serologic D neg. blood donors. Samples were tested in pools of 20 donors by PCR-SSP for RHD intron 4 or for RHD exon 4, exon 7 and exon 10. 21 RHD positive donors were detected in center A and 18 RHD positive donors in center B among the serologic D neg. donors. The molecular bases of the RHD positive samples were resolved in all cases. A total of 10 RHD alleles were novel: RHD(T201R, F223V, P291R) dubbed weak D type 4.3, RHD(I374N) dubbed weak D type 32, RHD(del147), RHD(del343), RHD(del449), RHD(del785), RHD(L153P), RHD(Y269X), RHD(IVS3+2T>A) and RHcE(1–3)-D(4–10). 13 samples in center A represented 5 known RHD alleles, most often RHDψ (n = 5), RHD(IVS3+1G>A) (n = 4) and RHD(M295I) in CDe (n = 2); 7 samples in center B represented the 3 known RHD alleles RHD(IVS3+1G>A) (n = 4), RHD-CE(2–9)-D2 (n = 2) and RHD(M295I) (n = 1). 9 donors in center A represented Del; in center B 9 were weak D and 6 Del; 13 of the remaining donors were confirmed to be D neg. despite carrying the RHD gene. We concluded that RHD genotyping of serologic D neg. donors at two facilities revealed carriers of the RHD gene expressing antigen D, albeit at low levels, in the range of up to 1:1,000 and 1:350 donors, respectively. At least 24 donors carried RHD alleles that were known or shown to express a weak D or Del phenotype. The RBC units donated by these donors may be capable of causing at least secondary anti-D immunization. This possible adverse clinical outcome was avoided by RHD genotyping each donor only once. Use of RHD genotyping would obviate the need to tightly control the sensitivity of serologic anti-D testing in blood donors. Further studies are needed to corroborate the current experience in particular in donors of non-white ethnic background. However, we think that RHD genotyping in first time blood donors has the potential to become a routine procedure in blood centers.