A recurring problem in recipient selection for deceased donor renal transplantation is a positive crossmatch in a “non-donor antigen sensitized “individuals. We initiated a study to determine what factors were causing this flow crossmatch positivity. There were 22 positive crossmatches studied out of 131 performed from 7-1-2011 to 6-30-2012. All of the potential renal transplant recipients demonstrated anti-HLA antibodies (N=131). None had demonstrable anti-HLA antibodies to the donor antigens at the time of crossmatch. We use pronase treated cells in a T and B cell flow crossmatch procedure with Mean Equivalent Standard Fluorescence (MESF) as a read out. Several crossmatches (n=13), (59 %) had IgM interference that was reduced with Dithithreatol (DTT). When DTT treated serum was used the crossmatches were negative. Five (N=5) were positive due to new anti-HLA antibodies that were present one or two months after the crossmatch was performed. One had heart procedures (VAD) that may have caused auto antibodies. One patient had a positive auto crossmatch indicating an autoimmune disease, such as lupus, that we suspect was causing the non-specific binding of IgG. There were two that we could not determine a factor for causing the positive flow crossmatch. In conclusion; 1) the use of DTT improves the identification of negative crossmatches, i.e.no demonstrable IgG anti donor antibodies. and 2) there are several factors that can cause positive flow crossmatches in “non-sensitized “individuals and support the need for actual flow crossmatching in tandem with the virtual crossmatch prior to transplantation.

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