P170 Correlating DSA with flow and cytometric crossmatch results: How old is too old?

Aim The goal of this study was to analyze crossmatch (XM) results among HLA DSA + patients, evaluating (1) locus-specific cutoffs, (2) additive effect of multiple DSA, and 3) confidence in XM prediction when single antigen testing is remote from XM. Methods 739 flow (FCXM) and cytotoxic XM (CDC) with known DSA were performed for routine clinical testing. FCXM u003e50 MCS (T cells) or 100 MCS (B cells) was considered positive, u003e200 MCS strongly positive. HLA antibodies were measured by single antigen.We used single DSA in same sample as the XM to “fit” the prediction model (MFI vs. median channel shift (MCS)), then evaluated multiple DSA and DSA in the most recent (but not the same) serum. Results When detected in the same serum used for XM, the MFI of DSA to HLA-A, B and DRB1 showed linear correlation with both FCXM and CDC. 83% of XM with Class I DSA 15,000 MFI were always resulted in strong positive FCXM and CDC. For HLA-B DSA, 76.9% at 3,000–5,000 MFI resulted in positive T FCXM, compared with only 37.5% of DSA to HLA-A. Antibodies to HLA-Aw4/Bw4/6 epitopes, or homozygous antigens, had a much lower threshold for positive T-FCXM. Only 33.3% of C DSA u003e5,000 resulted in positive T FCXM, compared with 100% of A and B. 100% of DSA to HLA-DR u003e3,000 MFI resulted in positive B-FCXM. For DQ DSA 8,000 MFI, 87.5% were positive. DSA to DP had an even higher MFI threshold to cause a positive FCXM (u003e10,000 MFI). Single DQ or DP DSA, even u003e15,000 MFI, were rarely positive in CDC. If either the highest, average or sum of multiple DSA I u003e5,000 MFI, 100% of T FCXM were positive. If the highest DSA was u003e10,000 MFI, FCXM were always strong. In general, the sum of the DSA was a better predictor of XM than average or iDSA. If DSA was detected within 3 months of the XM, results trended very similarly to when DSA was measured in the same serum. However, 18.5% of patients whose DSA was detected within 6 months of XM, and 24.3% whose DSA was seen more than 6 months prior, showed sufficient variation in DSA MFI over time to cause inaccurate XM prediction. Conclusions Risk assessment and donor selection are done to the best of the lab’s ability with available information at the time of organ offer. Solid phase antibody testing less frequently than semiannually risks inaccurate XM prediction due to fluctuation in DSA strength.