#OA2-AM25-MN-20 - The Growing Need for RH Allele Selected Red Blood Cell Units

The American Rare Donor Program (ARDP) registers blood donors with RH variant alleles lacking the high prevalence antigens. Donors with the same or similar RH alleles can be the source of compatible blood for Rh alloimmunized patients. RH allele selected (RAS) units are chosen based on comparing donor and patient RH alleles (Keller MA Ann Blood doi: 10.21037/aob-22-6).  We examined the growth in RAS requests received from 2011 to 2024 and analyzed the requests received in calendar year (CY) 2024.  


Study

Design/Methods: The ARDP database was examined for RAS requests by year. The number of donors with RH allele information and those matching the requests were enumerated. 

Results/Findings: The ARDP received less than 50 requests for RH allele selected units in 2011 compared to > 200 in 2023 and 2024 (Figure 1).  In 2024 there were 221 requests for 70 patients, with 22 (10%) cancelled within 1 business day (bd) and 36 (16%) cancelled due to death, discharge, units located in-house, or use of non-RAS units or patient refusing transfusion. The remaining 163 requests were for 46 patients, of which 63% had sickle cell disease. The patients had 21 different RHCE allele combinations, 35% of which expressed a conventional RhD antigen and 10% altered C antigen. Two patients were hr^S-^; four carried the r’s haplotype, two of which were group O homozygous r’s. Units were located for 13% of cases in 1bd, with 43% and 62% within 3 or 5bd, respectively. The number of requests per patient ranged from 1 (N=17) to 19 (N=1).  Eighteen patients had more than 500 donors classified as Tier 1 or Tier 2 (not carrying RH alleles predicted to be different from the patient). Of the 27 remaining patients, 12 had more than 1000 donors ranked as Tier 3 (similar alleles predicted to have similar phenotype). Twelve patients had less than 100 donors ranked Tier 1, 2 or 3; for these challenging patients, units were located within 23bd for all but one (hr^S- unit in 211bd) with 10 of the 42 units located in frozen inventory. Of the > 2500 donors with RH variant alleles in the ARDP, less than one-fifth had donated within the prior three years

Conclusions: There has been a significant rise in requests for RAS red cell units, including for patients on chronic red cell exchange protocols.  The ARDP filled 162 (82%) of 199 RAS requests in CY2024. Twenty of these were for patients carrying one or both RHCE*ce733G alleles and predicted to type  hrB+^vw/-.  Approximately two-thirds of requests were fulfilled within 5bd. The most difficult to fulfill RAS orders were for hr^S- red cell units.  The use of frozen inventory to fulfill difficult RAS requests suggests that blood centers are identifying and freezing red cell units from donors with RH genotype information. The number of donors in the ARDP with RH allele information has increased but donor retention is low, such that units from RH variant donors are not keeping pace with demand.