P153 Utility of the virtual crossmatch for living donor evaluations

Abstract

Aim Our institution performs 750–900 flow cytometric crossmatches (FCXM) annually in support of living-donor renal transplantation. The FCXM is costly, time consuming, and has interpretational conundrums (i.e., FCXM + donor specific antibody (DSA) negative or FCXM−/DSA+). By combining high resolution HLA typing with solid-phase antibody (SAB) testing, recipients should be transplantable based solely on a virtual crossmatch (vXM). Given the increased utilization of the vXM, our aim was to assess its utility in living donor evaluations. Methods We reviewed 100 living donor FCXMs and retrospectively performed a vXM for each pair. vXM was performed only on cases with available donor typing and SAB specificity testing. Additionally, ΔMESF values of the FCXM, FlowPRA results, and mean fluorescence intensity (MFI) values of DSA were analyzed. Results Initially, there was a 75% correlation between FCXM and vXM: 60 cases were negative for both FCXM and vXM; 15 cases were positive for both. However, 20 cases were FCXM+ (mostly B-cell) but vXM−, which tended to occur in non-alloimmunized recipients. 3/20 had autoimmune disease and 8/20 were transplanted without development of DSA. Although deemed compatible, the remaining 12 were not transplanted. These 20 cases were considered False Positive FCXMs. 5 cases were FCXM−/vXM+. Of these, 4 had Class I DSAs and 1 had both Class I and II DSAs; however, all 5 had low MFI values. (Table 1). Thus, reclassifying the false positive FCXMs, the final correlation was 95%. Download : Download high-res image (146KB) Download : Download full-size image Conclusions Though the initial FCXM/vXM correlation rate was only 75%, there are key characteristics such as differences in FCXM, ΔMESF, PRA values, and DSA MFI values that can help interpret discrepant result. Notably, there were 4 times more false positive FCXMs then false positive vXMs. Reclassifying the False Positive FCXM cases increased the correlation to 95%. These data support the utility of the vXM as a replacement for a physical FCXM for most living donor evaluations.