Diagnostic accuracy of Luminex lysate-based crossmatch MFI thresholds in predicting flow cytometry crossmatch outcomes: a large-scale analysis of 3711 donor-recipient pairs.

INTRODUCTION: Pre-transplant anti-HLA antibody detection techniques - have evolved from traditional cell-based assays to advanced solid-phase platforms with enhanced sensitivity. OBJECTIVE: MFI cut-offs for Luminex lysate-based crossmatch (LXM) assay lack consensus. This study evaluates the correlation between LXM-MFI values and flow-cytometric crossmatch (FCXM) results. MATERIALS AND METHODS: A total of 3711 pre-transplant donor-recipient pairs were analyzed using LXM and T- and B-cell FCXM. LXM-MFI values were evaluated against FCXM results using chi-square and ROC analyses to assess diagnostic metrics and odds ratios (OR). RESULTS: Of 3711 recipients (72.2% male and 27.8% female), Class I and II LXM were detected in 10.7% (396/3711) and 30.3% (1124/3711), respectively. Negative LXM results (MFI <500) correlated strongly with FCXM results. LXM class I weak positive category (MFI 500-1000) showed T-FCXM positivity in 26.2% (32/122; OR = 35.4; p < 0.001) while class II weak positives showed B-FCXM positivity in 13.4% (76/568; OR = 9.8; p < 0.001). Positive class I (MFI >1000) showed T-FCXM positivity in 81% (OR = 424.6; p < 0.001); class II (MFI >1000) exhibited B-FCXM positivity in 66.7% (OR = 127.7; p < 0.001). At MFI 1000 cut-off, class I LXM predicted T-FCXM positivity with 77% sensitivity, 98% specificity, and 96.8% accuracy, whereas class II predicted B-FCXM with 76% sensitivity, 94% specificity, and 91.9% accuracy. CONCLUSION: LXM-MFI cut-offs of <500 and > 1000 accurately predict FCXM outcomes for reliable risk stratification. Confirming FCXM testing remains crucial for weak positive LXM (MFI 500-1000) cases to optimize laboratory crossmatch workflows and avoid unnecessary donor exclusion.