Aims: To determine causes of Flow Cytometry crossmatch (FCXM) after a negative virtual crossmatch (VirXM), even with sensitive solid-phase antibody assay and complete HLA typings for deceased donors.

Methods: We reviewed single-center results of 704 FCXMs with 221 deceased donors for kidney or kidney/pancreas transplants from the past 2.5 years. Unacceptable antigens were determined with accumulative anti-HLA antibodies tested triannually. The guideline for positive reaction is MFI>1000 in single antigen beads (One Lambda, USA).

Results: Sixty-two (62/704, 8.8%) FCXMs were found to be positive or borderline positive. Only 4 (4/704, 0.57%) cases were true positive FCXM caused by DSA (donor-specific antibody) so transplantation wasn't performed. 3/4 cases were highly sensitized patients (cPRA≥95%) with allele-specific DSA later determined by high resolution typing of the donor. One (1/4) patient with a negative VirXM based on 3-month old sera had an unexpected 'true' positive FCXM with recent sera due to rebound of weak DSA as consequence of withdrawn immunosuppression after a failed 1^st transplantation. Fifty-eight (58/704, 8.2%) positive FCXMs were considered as “false positive” (FP) without convincing DSAs. Most FP FCXMs were caused either by autoantibodies (37/58, 64%) as indicated with historic/current positive autologous FCXMs or other unknown non-HLA factors (21/58, 36%). About half (30/58, 52%) of FP FCXMS were positive only in T cells, 15(26%) were positive only in B cells, 13 (22%) were positive in both T & B cells. Out of 27 patients whose sera gave PF FCXM, four patients contributed half (29/58) of FP cases because they were frequently cross matched. Some patients (7/27, 26%) has native autoimmune diseases. Thirteen patients were transplanted with FP FCXM on T cells only (n=8), B cells only (n=3), on both T & B cells (n=2). Seven transplanted patients demonstrated auto reactivity in current/historic sera. Five patients were highly sensitized (cPRA≥95%), 5 were moderately sensitized (cPRA=20-95%) and 4 were low sensitized (cPRA<20%). No hyper acute antibody-mediated rejection and graft lost were found in all 13 transplants.

Conclusions: This is 1^st detailed analysis about PF FCXM with deceased donors in VirXM era. When conservative approach of defining unacceptable antibodies is used, transplantation based on negative VirXM is safe in most cases, regardless the result of FCXM or level of cPRA. Caution shall be given to patients with allele-specific antibodies or recent allosensitization.

CITATION INFORMATION: Xu Q, Leckie S. Positive Flow Crossmatch After Negative Virtual Crossmatch -A Single Center Experience. Am J Transplant. 2017;17 (suppl 3).

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