(P-IG-1) Estimating Serological Underdetection Rates of Patients with Weak and Weak Partial RHD Genotypes

Background/Case Studies: For patients with weak RhD RBC phenotypes, RHD genotyping is recommended to determine need for Rh Immune Globulin and D-negative RBCs. Across the variant D antigen-strength spectrum, very weak phenotypes may type D-negative and modestly weakened phenotypes may type D-positive. Serological detection rates for the most common weak D alleles are incompletely characterized and would help better assess anti-D risks in weak partial-RHD patients.

Study

Design/Methods: Four US studies employing RHD genotyping in patients with weak D phenotypes provided racial and ethnic data for calculating serologically detected prevalences of weak RHD variants in 35,000 Whites (W), 38,000 Hispanics (H) and 14,000 Blacks (B). Three studies used microplate and 1 used gel & tube typing; 2 had anti-D data. We obtained allele frequencies of single-nucleotide variants (SNVs) in weak D types 1, 2 and 3 from the Genome Aggregation Database (gnomAD v2.1.1) (106,000 European, 33,000 Latino alleles) and adjusted them for overcount of hemizygous SNVs as homozygous. We compiled weak partial RHD allele frequencies in 6 published US RHD-genotyped cohorts of B subjects or sickle-cell-disease patients (9326 alleles). From RHD variant allele frequencies, we calculated the predicted genetic prevalences of patients with hemizygous or homozygous RHD variants (Ouchari 2018 formula) using US D-negative allele frequencies 0.41 for W, 0.27 for H and 0.29 for B (Kacker 2015). The ratio of serological prevalence to genetic prevalence yielded the serological detection rate.

Results/Findings: Estimated detection rates for weak D types 1, 2 and 3 were 18% (95% CI 12-25%) in W and 13% (6-33%) in H. In B detection rates were 28% (17-46%) for RHD type 4.0 and 26% (17-39%) for all other represented weak partial RHD variants combined. However, the DAR detection rate was 80% (38-156%). Figure A plots variant detection rates vs RhD antigens/RBC. At lower and upper bounds of reduced RhD/RBC, type 2 and DAU5 were detected serologically only by gel-tube discrepancies. Detection rates were statistically equivalent in gel-tube vs. microplate for combined weak D types 1-3 (W, 28% vs 17%) and non-4.0 partial RHD (B, 32% vs 23%) (both p>0.24). Type 4.0 detection was higher in microplate (B, 9% vs 39%, p< 0.001). Anti-D was present in 11% of identified B weak partial RHD cases, but only 3% of total projected cases.

Conclusions: Based on allele frequencies, most weak and weak partial RHD variants are not detected serologically. Differences in typing methods and individual antigen expression contribute to this disparity. DAR appears more readily detectable serologically than other weak D variants. Adding one-time tube typing may enhance detection of weak partial-RHD phenotypes in selected patients. D+ RBC exposure data are needed for immunogenicity analysis, but weak partial-RHD patients who form anti-D represent a very small fraction of all such variants.

Importance of research: Our innovative analysis includes compilation of weak-RHD allele frequencies from gnomAD and weak partial-RHD allele frequencies from 6 US black or sickle-cell-disease cohorts. We also report that gnomAD overestimates many RHD SNV allele frequencies by double-counting hemizygotes as homozygotes. RhD immunogenicity estimation in weak partial RHD patients will need significant adjustment for underlying genetic prevalence.