Very low residual concentrations of rituximab long after infusion still induce positive B-cell complement-dependent cytotoxicity-crossmatch.

Rituximab may induce positive B-cell complement-dependent cytotoxicity crossmatch (CDC-XM) in the absence of donor-specific antibodies, as we report in these two cases. We retrospectively assessed the in vitro concentration-effect relationship of rituximab in sera. B-cell CDC-XM results were positive only in the presence of rituximab, even with low concentrations (inferior to 1 μg/mL).