Expanding donor options: haploidentical transplant recipients also highly likely to have a 7/8 matched unrelated donor.

The use of haploidentical related donor (HRD) hematopoietic cell transplants (HCT) in the United States (US) grew >4x in the last decade, driven mainly by use of post-transplant cyclophosphamide (PTCy) based graft-versus-host-disease prophylaxis. However, not all patients have a suitable HRD available. In this study, we explored the existence of unrelated donors (URDs) on the NMDP Registry at the 8/8 or 7/8 match level for patients receiving HRD HCT in the US and reported to the CIBMTR from 2013-2020. The data consists of 9696 HRD HCT recipients aged 0-87. The NMDP Search Prognosis score and a Search Simulation were run for each recipient to calculate estimated counts of 8/8 and 7/8 matches on the registry. NMDP search prognosis score varied by patient ancestry, with 27.5% non-Hispanic white HRD recipients having a good score compared to 4.6% of African-American recipients. Overall, 34% of recipients had ≥18/8 URD and 84% had ≥17/8 URD. Recipients of older HRDs (age >35) had between 20-65% likelihood of ≥5existing 7/8 URDs age ≤35. Donor selection practices varied among the 10 highest volume HRD centers, where 6 had >20% chance of an existing 8/8 URD for their HRD recipients whereas 4 had low likelihood of identifying an 8/8 URD. In conclusion, while most US patients undergoing HRD HCT do not have an existing 8/8 URD, the majority have an existing 7/8 URD. Studies comparing outcomes in patients receiving either HRD or 7/8 URD HCT and PTCy-based GVHD prophylaxis may be warranted.