P129 Dealing with false-positive virtual crossmatch in lung recipients

Abstract

Aim Virtual crossmatch (VXM) has increased non-local organ utilization and improved clinical outcomes for sensitized lung patients. However, the concordance between VXM and cell-based crossmatch is not absolute because Single Antigen Bead (SAB) assay is prone to both false-negative and false-positive reactions. Since false-positive antibody specificities may unnecessarily deny an organ based on VXM, the goal of this study was to evaluate what additional testing may be warranted to recognize such reactions and to improve virtual crossmatch accuracy. Methods All sera were pre-treated with EDTA to inactivate complement in order to avoid prozon-like effect and were analyzed by SAB. A positive VXM was defined as the presence of donor specific antibodies based on SAB that would result in unacceptably positive Flow Crossmatch (FCXM). FCXM was done using 3-color analysis on a 1024 channel scale. All sera was retrospectively tested by FlowPRA Screen and LSPRA (phenotype bead) assays to rule out antibodies against denatured HLA epitopes detected by SAB. Results Of 58 consecutive VXM performed during July-December 2016 for lung candidates with CPRA > 10%, 28 had no DSAs or had acceptably weak DSAs and were predicted to result in negative or acceptably low positive FCXM. Other 30 VXM had one or more strong DSAs and were predicted to result in unacceptably positive FCXM and the organ offers were refused. Additional antibody testing showed that only 23 out of 30 VMXs should have been called positive. The other 7 VXM were called unacceptably positive due to the presence of antibody against denatured antigens. Three patients had antibodies against class I denatured antigens (MFI ranging 2500–3500) and four patients had antibodies against class II denatured antigens (MFI ranging 2000–14,000). After using LSPRA and FlowPRA Screen assays, the antibody against denatured antigens were removed from the list of UA and 5 out of 7 patients were successfully transplanted. Conclusions We found that at our center 12% VXM are false-positive due to the presence of antibodies against denatured antigens detected by SAB. Multiple antibody testing platforms, including SAB, LSPRA, and FlowPRA Screen, can be used for antibody characterization to avoid erroneous assignment of UA that may lead to an unnecessary organ offer refusal.