RHD Genotyping to Resolve Weak and Discrepant RHD Phenotypes: The “Serenissima” Experience

Background: A considerable number of RHD alleles responsible for weak and partial D phenotypes have been identified. Serological determination of these phenotypes is often doubtful and makes genetic analysis of RHD gene highly desirable in transfusion recipients and pregnant women.

Aim: We report the experience of Mestre Blood Bank in analysis of the RHD gene in six years from 2018 to 2023.

Methods: Subjects for RHD gene analysis were selected for presence of a serological weak D phenotype, defined as reactivity of RBCs with an anti-D reagent giving no or weak (≤2+) score in initial testing but agglutinating moderately or strongly with anti human globulin (AHG). These samples were selected for genotyping using the microarray-based method Bead-Chip supplied by Werfen.

Results: From 2018 to 2023, we selected, for RHD gene analysis, 555 subject with D weak phenotype; 86 subjects (15.5%) were D positive and 56 (10.1%) were D negative, without variant, in 413 subjects a D weak or a D variant was observed.

Discussion: Many serological weak D phenotypes are associated to RHD gene mutations leading to one or more amino acids substitutions in the RhD protein predicted to be within or below the RBC membrane, causing decreased antigen expression on the red cell surface. Prevalence of serological weak D phenotypes varies by race and ethnicity. Serological weak D phenotypes are the most common D variants detected in Caucasians (0.2%-1.0%). The majority, as in our series, are associated with weak D type 1, 2 or 3. Our data confirmed a high prevalence of weak D type 1 and type 2, but we observed a high prevalence of type 11 and 15 and of the uncommon type 18 too. The most common partial D phenotypes in Europe are DNB, DVI, and DVII. Our data confirmed a high prevalence of D partial type VI. Studies indicate that D partial transfusion recipients are at risk of forming alloanti-D when exposed to conventional RhD-positive blood units.