*Purpose: Flow cytometry crossmatching (FXM) is recognized as a valuable tool to assess immunological risk prior to renal transplantation. The increased resolution in HLA typing and enhanced sensitivity for HLA antibody detection has promoted the widespread use of virtual XM. Our aim was to correlate virtual FXM predictions with observed FXM results and assess outcomes for patients with discordant predictions.

*Methods: We performed a retrospective analysis of the last 560 deceased donor kidney crossmatches performed for our center from January 2017 through October 2018. We compared virtual predictions derived from patient antibody history and recipient/donor HLA typing with actual FXM results. We evaluated graft function and biopsy proven acute rejection (BPAR) for patients transplanted with discordant XM results.

*Results:

Of the 560 FXM, 44 did not match prediction (7.86%). Four were attributed to undisclosed sensitizing events (2/44) or absence of allele level donor HLA typing (2/44). We identified 6/560 as false negative (“FN”; DSA present but FXM negative) and 34/560 as false positive (“FP”; DSA negative but FXM positive). For all 560 cases, the patients had negative cytotoxic XM results. The 6 FN results were observed in the presence of borderline-low DSA levels; 4/6 patients were transplanted. Of the 34 FP results, 15 were Flow T-cell positive only (FT+); 6 were Flow B-cell positive only (FB+) and 13 were Flow T- and B-cell positive (FTB+). One deceased donor had two independent patient results that did not match prediction. One patient was T and B flow positive, one was T-cell flow positive only (Table 1). Of the 44 discordant cases, nine patients were transplanted (4/9 FN; 3/9 FP; 2/9 after auto-XM correction). No patient experienced BPAR. Two patients experienced early non-HLA related graft losses (one case of recurrent IgM dominant immune glomerulonephritis and the other to accelerated hypertensive changes). One patient with evidence of C4d positivity on biopsy also received an ABO A2 to B transplant. One patient died from non-HLA related causes with a well-functioning allograft.

*Conclusions: Current HLA antibody testing and virtual XM predictions are greater than 92% successful. In cases of discordant results, patients may still have acceptable outcomes without early AMR.

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