Clinical Importance of Anti-human Leukocyte Antigen-specific Antibody Concentration in Performing Calculated Panel Reactive Antibody and Virtual Crossmatches

Background: Highly sensitized patients develop multi-human leukocyte antigen (HLA) specific antibodies. This study measures concentrations of anti-HLA antibodies in multispecific sera by converting fluorescence intensity into molecules of equivalent soluble fluorochrome (MESF) units. This was used to determine MESF units required for a positive T and B flow crossmatches (FLXM).

Methods: MESF values of negative controls and sera from patients devoid of HLA antibodies were measured by FLXM and flow panel reactive antibody (PRA) screening beads. Fluorescence intensity values of anti-HLA specific antibodies determined by FlowPRA single antigen beads of highly sensitized patients were converted into MESF units. In addition, endpoint titers, MESF units, and % PRA of 26 sera were established.

Results: MESF analysis accurately predicted the outcomes of 100% of T and B FLXM of sera with strong (MESF units>18,000) donor-specific antibody (DSA). The predictive values of T and B FLXM declined to 95% and 88% with weak DSA (6,000 MESF<10,000). Endpoint titers of sera from highly sensitized patients ranged from 1:512 to 1:8 with corresponding MESF values of 452,596 to 20,000 units. However, there was no statistical difference in PRA values among these sera (95%-100%). We successfully transplanted five patients who had weak donor-specific HLA antibodies (MESF units>2,000). The graft survival at 1 year was 100% and there was no evidence of DSA posttransplant.

Conclusion: MESF analysis is both a time and cost efficient way of measuring antibody strength. The strength of the antibody present in the sera of transplant candidates is critical for crossmatch prediction.