While establishing a flow cytometry crossmatch (FCXM) exchange program for laboratories in solid organ transplantation we conducted a wet bench feasibility study during a national conference in Germany. Aim of this first step was to acquaint participating laboratories in the FCXM procedure. Especially the use of pronase and proper controls was in scope. Currently, the complement dependent cytotoxicity (CDC) assay is the standard method used for crossmatching in solid organ transplantation. CDC is based on laboratory experience and does not comply with the IVDR requirements (in vitro diagnostic medical devices and repealing, Directive 98/79/EC Regulation (EU) 2017/746, consolidated 23/03/2023), which prompted the laboratories to change their policies.

17 participants from 9 different institutes were present, and the Halifaster FCXM assay (https://doi.org/10.1016/j.humimm.2017.10.020) was demonstrated in two practical sessions. The use of pronase to eliminate B cell background and to limit the false positive and false negative reactions were demonstrated. In addition, a set of negative and positive control sera was featured that together serve to proper control for washing technique and ensure the validity of the FCXM results.

The results indicated that the use of pronase treatment is essential to decrease B cell background and to limit both false positive and false negative B cell reactions in FCXM. Furthermore, a carefully selected and developed set of negative and positive control sera was featured and demonstrated the importance of using proper controls to ensure proper washing technique and the validity of the FCXM results.

The Halifaster FCXM method is rapid, cost effective, provides high quality results and is used by many laboratories internationally for pre-transplant crossmatching. Our next step is to conduct a large study with many participating laboratories in central Europe to evaluate the Halifaster FCXM protocol as a replacement method for the CDC crossmatch. We aim to accomplish this by end of August 2024.

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