Introduction: A negative crossmatch (XM) is critical for solid organ allocation. Flow cytometric XM (FCXM) is more sensitive than complement-dependent cytotoxicity XM (CDC-XM), and has become the standard assay. The virtual crossmatch (vXM), which uses recipient antibody specificity from solid-phase assays and donor typing from molecular methods to determine the presence of donor-specific antibodies (DSA) in silico, is used to supplement, or replace, physical XMs. Discordant results between a physical and virtual XM are common, however, and can lead to uncertainty of the risks for a particular transplant. Here we analyzed positive FCXM results from patients without DSA to determine if vXM results are a reliable substitute for such patients.

Methods: 17 solid organ transplant candidates with at least one positive physical XM in the absence of DSA, tested between 3/2013 and 12/2016, were studied. HLA antibodies were identified by the University of Chicago Medicine (UCM), and prospective FCXM on pronase-treated cells were performed by Gift of Hope (GOH). FCXM on untreated cells and CDC-XMs were performed by GOH, and, if donor cells were available, FCXM on pronase-treated cells was repeated at UCM. Recipient sensitization history, infection status, and/or medical condition relevant to FCXM reactivity were reviewed.

Results: Of the 17 patients with false-positive FCXM results, 10 were T cell, 13 were B cell, and 6 were both B and T cell positive. These patients were enriched for autoimmune diseases (8/17) and viral infections (3/17) known to interfere with FCXM. Of those tested by FCXM with untreated cells, 4/5 resulted in negative XM results, while 3/4 tested by FCXM at UCM had negative XM results. All of the patients (5/5) tested with CDC-XM were negative.

Conclusion: The crossmatch assay has undergone several changes over the last 50 years. While the most recent refinement, vXM, is used for all UNOS organ allocation, physical XMs are also routinely run. Physical XM assays, however, are not standardized, and individual labs may tune their XM for greater sensitivity, resulting in more frequent false-positive results. The data presented here show that several variations of a physical crossmatch may be required to verify results of a negative vXM, complicating organ allocation. In cases where a physical XM result does not correlate with the vXM, the vXM and other clinical information can be relied upon to determine an accurate immunological risk to a patient.

CITATION INFORMATION: Brown N, Xin J, Weidner J, Upchurch R, Marino S. False-Positive Flow Cytometric Crossmatch Results and the Utility of the Virtual Crossmatch. Am J Transplant. 2017;17 (suppl 3).

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