P004 The challenge of virtual crossmatching in the presence of weak donor specific antibodies

Abstract

Aim Our laboratory uses a normalized 2500 MFI cut off in the Luminex single-antigen bead assay to define unacceptable antigens. Antibodies (Abs) between 1000 and 2500 MFI are considered weak positive, but are not used to strictly rule out potential donors. The challenge is to accurately predict flow crossmatch (FXM) results in the presence of weak donor-specific Abs (DSA). Method We identified 62 FXM performed in the presence of weak DSA. For simplicity we considered only the B-cell FXM results. We analyzed the data based on the DSA MFI values or DSA epitope profile (DSA against shared epitopes (SE), DSA against a single antigen (Ag), and multiple weak DSA (M)). Results 39/62 (64%) were positive and 22/62 (36%) were negative. All (10/10) FXM with multiple weak DSA were positive. The majority (13/17) of FXM in the presence of weak DSA against a single antigen (Ag) were negative (76%). On the other hand, most (25/34) FXM with weak DSA that shares epitopes with other antigens (SE) were positive (74%). Table 1 summarizes these results. When we analyzed the FXM results based on DSA MFI (excluding multiple DSA cases), we observed that DSA with MFI of 1,000-2,500 resulted in a positive FXM in 50% of cases (Table 2). Unexpectedly, we got more positive FXM (14) than negative (7) when the DSA was under 1,000 MFI. Further analysis revealed that 13/14 of these positive FXM were caused by Abs with reactivity to SE and in particular Abs against HLA-B and -DRB1. Conclusion MFI values of weak DSA are not a reliable predictor of FXM results. When performing a virtual XM in the presence of weak DSA, the Ab profile is a better predictor for FXM reactivity (PPV for DSA with SE is 73.5, independent of the MFI value). Abs that target broadly-shared epitopes frequently cause positive FXM. Download high-res image (105KB) Download full-size image Download high-res image (104KB) Download full-size image