*Purpose: Calculated panel reactive antibodies (cPRA) is used by the United Network for Organ Sharing (UNOS) to quantify degree of HLA sensitization and assign priority in organ allocation. The current calculator relies heavily on low-resolution unacceptable antigen (UA) assignment and disregards the DQA, DPA, and DPB loci. We calculated cPRA based on 11-locus high-resolution genotype frequency data from the National Marrow Donor Program (NMDP), and compared it with the standard UNOS cPRA to determine whether it would change assigned allocation points in a cohort of active waitlist patients at our center.

*Methods: NMDP calculator: http://transplanttoolbox.org/nmdp_cpra/

Antibody specificities from single antigen bead assays on 420 consecutive active waitlist transplant candidates were used to compute UNOS and NMDP cPRA values. All positively reacting beads from the most recent assay were considered UAs, and were entered into the UNOS calculator at antigen-level and into the NMDP calculator at two-field allele-level with DQ and DP entered as alpha/beta combinations. Allocation points were determined based on the current UNOS KAS sliding scale.

*Results: 214/420 patients exhibited HLA antibodies, and thus were impacted by changing calculators. In most cases, the NMDP analysis resulted in lower cPRAs and allocation points. Patients in the 50-79% UNOS cPRA group showed the most cPRA variation, while allocation points varied the most within the highest cPRA groups (Figure 1). Importantly, 10/35 patients assigned to the 100% UNOS cPRA group dropped to a lower cPRA group using the allelic calculator, with 5/10 dropping 2 or more groups. 8/10 were primarily due to the use of HLA-DQ alpha/beta allelic combinations rather than antigen. Inclusion of DP had a varying effect on NMDP calculations.

*Conclusions: The use of allele-level UAs and inclusion of all HLA loci in cPRA calculation resulted in prominent changes to the allocation points assigned to current waitlist patients, particularly those that are highly sensitized. Currently, it is possible to assign UAs at the allele level; however, the UNOS calculator does not translate this into changes in cPRA nor in allocation points. Capturing the true level of HLA sensitization is critical, as minor decreases in cPRA at the highest end translate into significant increases to the pool of compatible donors. Our data suggest that some patients may be unjustly prioritized within the allocation system while missing on potential donors.

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