Soft kidney allocation improves HLA matching and maintains allocation order

Kidney allocation scores aim to balance transplant efficacy and equity but often deprioritize Human Leukocyte Antigen (HLA) matching, leading to reduced graft lifespan and a higher risk of donor-specific antibodies (DSA). This study introduces a "soft allocation" strategy to improve HLA match rates while minimally disrupting the allocation queue or equity.

Donors include directed-living (D-LD), non-directed-living (ND-LD), and deceased (DD) types. In the "soft allocation" system, instead of automatically assigning the kidney to the first person in line, the kidney is offered to the best HLA match among the top patients on the waiting list. The predefined size of the top patient group (typically 5-20 patients) is referred to as K. This approach preserves blood type and age compatibility while protecting patients with rare HLA types.

Analysis of 95,250 US DD transplants shows an increase in HLA-A&B&DR match scores from 1.45 to 1.77. In 42 recent DD cases and 230 ND-LD transplants in Israel, match scores improved from 2.33 to 2.54 and from 1.34 to 2.03, respectively. Similar enhancements are observed in related donor-recipient pairs using crossover transplants, achieving a three-fold match improvement without worsening any recipient's match.

For predefined allocation rules, soft allocation enables significant HLA matching improvement with minimal impact on allocation order, benefiting both unrelated and related transplants. Future work will extend this approach to include sensitized donations.