– Clinical Instructor, MedStar Georgetown University Hospital
Aim: Accurate prediction of flow cytometric crossmatch (FCXM) results remains challenging in patients with low-level donor-specific HLA antibodies (DSAs) targeting shared epitopes. This study aimed to determine DSA levels associated with positive FCXM results.
Methods: Sera with low-level HLA antibodies (MFI 0-3000) reactive to shared epitopes (Bw4, A2 CREG, A19 CREG, DR52 group, and DQ3) were analyzed. Antibody specificities were confirmed using multiple platforms including single antigen and phenotype bead assays. Surrogate FCXMs were performed using donor cells expressing relevant HLA antigens. Additional donors expected to result in a negative crossmatch due to the absence of DSAs were selected. Median channel shift (MCS) values were correlated with DSA MFI using linear regression analysis.
Results: All DSA-negative FCXMs were negative. Of 22 DSA-positive FCXMs, eight T-cell and nine B-cell crossmatches were positive. Multiple DSA-positive FCXMs were below our positive cutoff but had elevated MCS, indicating some low-level DSAs insufficient to yield a positive result based on our laboratory’s cutoff (T-cells: 70 MCS/B cells: 80 MCS). Regression analysis estimated that DSA MFI thresholds of ~1500 for T-cells and ~2000 for B-cells were associated with FCXM positivity (Figures 1 and 2).
Conclusion: These findings enhance virtual crossmatch interpretation and may guide perioperative and post-transplant management, including immunosuppression and early post-transplant DSA monitoring due to the risk of a memory response.
