A Flow Cytometry Crossmatch With Combined Cytotoxicity and <scp>IgG</scp> Detection: Ten Years of Use in Deceased Donor Kidney Allocation

Virtual crossmatch (VXM) is increasingly used in deceased-donor kidney allocation, raising questions about the added value of a physical crossmatch (PXM) when donor-specific antibody (DSA) are absent. We applied our previously described flow cytometry-based assay (FCtox), combining complement-mediated cytotoxicity and IgG binding, across 3236 routine crossmatches performed between 2018 and 2024. All donor offers underwent VXM followed by prospective PXM. Among 704 patients, 95% of FCtox assays were performed in VXM- cases. FCtox positivity was uncommon in VXM- cases (9%), and overall concordance between VXM and FCtox was high (89%). Further analyses indicated that isolated FCtox abnormalities in VXM- cases primarily reflect patient-specific background reactivity, including non-HLA or IgM-mediated effects. Among transplanted VXM- patients with FCtox positivity (n = 12), no early antibody-mediated rejection occurred. In VXM+ patients, DSA mean fluorescence intensity (MFI) values above 5000 were associated with increased likelihood of FCtox IgG positivity, although overlap between groups limited the ability of MFI alone to predict FCtox IgG reactivity. Overall, VXM reliably identified clinically meaningful immunologic risk, while FCtox added little value in VXM- candidates but remained useful in selected VXM+ or discordant scenarios. These findings support a VXM-driven allocation strategy with selective rather than routine use of PXM.