*Purpose: Despite implementation of virtual cross matches, flow cytometry cross matches (FCXM) are still used by many transplant centers to determine immunological risk prior to kidney transplantation. Sensitivity and specificity of FCXM are better than these of complement-dependent cytotoxicity cross matches. Nonetheless FCXM is still subject to false positivity, which may prevent patients from receiving an otherwise acceptable life-saving organ transplant. Other than anecdotal reports in lupus, characteristics of patients with false positive FCXM remain undefined. To determine if common profiles of patients prone to false positive FCXM exist, we examined the demographics and native diseases of patients with positive autologous and false positive allogeneic FCXM.

*Methods: We reviewed autologous and allogeneic FCXM performed in our center (2014-2018). FCXM were performed with conventional methods using Ficoll-purified cells. After May 2015, modified Halifax protocol and cells purified with magnetic beads were used. All FCXM were performed with pronase-treated peripheral blood lymphocytes. Any positive (>3SD) or borderline (>2SD) FCXM in at least one tested serum were considered as positive.

*Results: We studied 530 patients waiting for heart, kidney or any kidney-combined organ transplant. Improvements to FCXM and cell isolation methods significantly reduced the positive rate from 14.6% to 5.3% for autologous FCXM. Patients with native diseases considered ‘immunological’ (vasculitis, lupus, IgA nephropathy) had more positive autologous FCXM (OR=3.31, p=0.003) vs. patients with all other diseases. In multivariate analysis, kidney patients who were tested using our updated method (n=321) still showed that these immunological diseases were a significant predictor for positive autologous FCXM (OR=4.79, p=0.006). Interestingly, patients on peritoneal dialysis (PD) also had significantly more positive autologous FCXM than patients on hemodialysis or waiting for pre-emptive kidney transplants (OR=3.27, p=0.02). Age, gender and PRA were insignificant predictors. Our findings were confirmed in patients who had false positive allogeneic FCXM with deceased donors. Twenty of 24 (83.3%) patients with false positive allogeneic FCXM tested with our updated method either had immunological diseases originally or were on PD. Reasons for the higher false positivity in PD patients are unknown although prevalence of Th2 immunity and/or recent or subclinical peritonitis in PD patients may have contributed.

*Conclusions: Our study indicates that along with autoimmune diseases, patients with IgA nephropathy, or those on PD are also prone to false positive FCXM. This is helpful when interpreting an unexpected positive FCXM, especially for time-sensitive transplantation from deceased donors.

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