175-P: NEGATIVE CDC AND FLOW CROSSMATCH BUT STRONG DSA DETECTED BY LUMINEX SINGLE ANTIGEN TEST AND C1q SINGLE ANTIGEN TEST

Aim Virtual xm and post-transplant (tx) monitoring by solid phase single antigen test are widely used by many transplant centers to identify compatible donors. The aim of our study was to determine the basis for the detection of strongly binding DSA by solid phase assays with concomitant negative flow and CDC xms. Methods Pre- and post-transplant sera from an allograft recipient were tested for DSA by xms (Flow and CDC), LSA IgG including a 1:8 dilution, C1q assays, and HLA typing was performed by SSO (Thermo-Fisher Scientific). Results A 63 yr. old female allograft recipient received a 0 mismatched DD graft disparate only for DQA1∗01:03/DQB1∗06:03. Negative flow and CDC xms indicated no detectable DSA. Pre and post tx LSA detected significant DSA binding (>8500 MFI) and C1q (> 2500 MFI) binding for DQA∗01:03/DQB∗06:03. This DQ6 antibody specificity was confirmed by the reagent vendor. CDC antibody screens were negative with the 6 DQ6 positive panel cells. Flow and CDC xms remain negative even with a 1:8 dilution. Donor DQB1 typing was confirmed by sequencing methodology. The patient continues to have good post tx allograft function. Conclusions This case exemplifies the continued need for cell based testing as an adjunct to solid phase testing. The DQ6 flow xm needs to be further confirmed by DQ6+ surrogate cell line or recombinant cell line testing. Posttx cell based xms appear to be a better monitoring tool than solid phase assays. Several questions remain to explain these results. Is this antibody functional? Are denatured antigens being detected by the solid phase assays. Is the antigen density on the beads higher than antigen expressed on lymphocytes, and especially for HLA-Cw, DQ, and DP antigens? How do these results influence the assignment of unacceptable antigens?