Background: Flow crossmatching(FCXM) has improved detection of low level anti-HLA antibody. However, FCXM doesn't accurately discriminate anti-Class I vs. Class II antibody, may detect non-HLA antibodies and may increase false positives, particularly against B cell targets. OneLambda has developed a novel bead based assay (Flow DSA-XM) that specifically identifies antibody directed against Class I, DR and DQ and may reduce rate of false positive XMs. This report summarizes our initial comparison of DSA-XM results with conventional FCXM.
Methods: Positive/negative ranges for DSA-XM(mean±3 SD) were established by testing 5 sera from  0% PRA male patients against 5 normal PBls. FCXM ranges were already established in lab.  FCXM and DSA-XM were performed on serum from 30 waitlisted renal patients against 11 donors and compared with DSA results using luminex (Immucor).
Results: Positive/negative means and ranges for DSA-XM Class I showed good separation and were comprable to T-FCXM. In contrast, separation between positive and negative controls for both DR and DQ were very narrow compared to B-FCXM.

We then explored correlation between XMing and identified DSA using sensitivity and specificity analysis. T-FCXM and DSA-XM Class I showed identical sensitivity (71.4%) and specificity (83.3%). In contrast, DSA-XM DR showed lower sensitivity (93.7% vs. 62.5%) but improved specificity (54.5% vs. 100%). DSA-XM DQ also showed reduced sensitivity (88.2% vs. 41.2%) but slightly improved specificity (36.4% vs. 63.6%) compared to FCXM. Lastly, attempts to correlate DSA-XM levels with DSA strength (MFI) using linear regression showed poorer correlation for Class I (r²=0.5), DR (r²=0.3) and DQ (r²=0.1) than FCXM (r²=0.75).
Conclusion: DSA-XM is an ingenious approach to increasing discrimination of non-HLA antibodies from anti-Class I, DR and DQ antigens. However, the minimal separation between positive/negative Class II controls makes identification of positive crossmatch challenging. In addition, our initial evaluation indicates DSA-XM, particularly against ClassII, underestimates low level DSA and may result in higher incidence of false negative crossmatches. These initial findings, plus high cost of the kit and short shelf-live indicates additional studies are needed regarding how, or if, this assay should be incorporated into laboratory practice.