Related and Unrelated Donor Match Presence for Haematopoietic Stem Cell Transplantation in a Portuguese HLA Referral Cohort

Identifying a suitable HLA‐matched donor is essential for allogeneic haematopoietic stem cell transplantation (HSCT), but match presence depends on donor source, family workup, matching thresholds, and donor‐dataset representation. We retrospectively analysed fully anonymised records from a Portuguese referral cohort (2021–2026), comprising 924 patients with complete six‐locus HLA data, evaluated relatives, and Portuguese Bone Marrow Donor Registry records. HLA‐A, ‐B, ‐C, ‐DRB1, ‐DQB1 and ‐DPB1 assignments were harmonised at high resolution, retaining P groups without conversion to specific two‐field alleles. Related donor match presence was evaluated at 12/12, ≥ 11/12, ≥ 10/12 and probable haploidentical levels, defined by allele sharing at all six loci without confirmed haplotypic phase. Unrelated donor thresholds were 8/8, ≥ 7/8, 10/10, ≥ 9/10, 12/12 and ≥ 11/12. Population‐background descriptors, HLA frequency patterns, and exploratory prediction models were also examined. Among all patients, 185 (20.0%) had a 12/12 related donor and 596 (64.5%) a probable haploidentical relative; among 653 patients with an evaluated relative, these proportions were 28.3% and 91.3%. Unrelated donor match presence was 13.1% at 8/8, 48.5% at ≥ 7/8, 12.8% at 10/10, 36.5% at ≥ 9/10, 2.9% at 12/12 and 12.4% at ≥ 11/12. In a same‐pool analysis of 23,004 HLA‐DPB1‐complete donors, match presence decreased from 10.5% (10/10) to 2.9% (12/12), and from 31.8% (≥ 9/10) to 12.4% (≥ 11/12). Strict 12/12 identity was descriptive, not a clinical donor‐selection target. Donor match potential is best characterised through structured search profiles integrating family workup, HLA thresholds, HLA‐DPB1, and dataset representation. These findings describe genetic match presence within analysed datasets, rather than clinical donor availability.