Purpose: Prior to KTx, a XM is performed between the intended recipient and donor. This XM can be positive despite no DSA. It can be difficult to interpret the clinical significance of such findings. This study investigates the outcomes of transplants performed with this XM result.

Methods: All patients (pts) who underwent KTx at our center between 2009 and 2013 were examined. DSA was evaluated using solid phase methods. Flowcytometric XMs were performed and considered positive if the T cell Mean Channel Shift (MCS) =/>90 or the B cell MCS is =/>120. Any positive T or B cell XM in the absence of DSA was included. No pt received any type of desensitization prior to transplant.

Results: 13 pts had a positive XM with no DSA. There were 11(84.6%) females and 2(15.4%) males; all were African American (84.6%) or Hispanic. 6 pts (46.2%) had an autoimmune disease (lupus, sarcoidosis) or HIV, and 5 had a previous KTx. 6 patients had a +T cell/-B cell, 1 had a –T cell/+B cell, and 6 had a +T cell/+B cell XM. MCS for positive T cell XMs ranged from 87-495 (mean 163), and for B cell 128-339 (mean 212). Of these 13 pts, 2 (15.4%) developed post-transplant de novo DSA. One had a positive auto-XM, and the other had a negative one pre-KTx. Post-KTx, 1 had biopsy proven antibody mediated rejection (AMR) and was treated with plasmapheresis, IVIG, Rituximab, and Bortezomib with continued good graft function. 1 other pt with no DSA had rejection, clearly due to medication non-compliance. In these 13 pts, there was 100% 1 year graft and patient survival.

Conclusions: Our center's policy has been to accept organs for pts with a positive XM and no DSA, particularly if their auto-XM is positive, as can often be found in pts with autoimmune disease or HIV. 15.4% of pts with a positive XM and no DSA developed de novo DSA after KTx. In our center, we divide our recipients into antibody risk categories. Risk of de novo DSA for the +XM/no DSA pts (15.4%) was comparable with our low risk pts (15.3%). For comparison, our moderate risk pts (those with DGF, slow graft function, child to mother transplant, or prior transplant failure due to rejection) was 27-33%. These findings confirm that grafts from XM positive donors in the absence of DSA can be used with great success, particularly with an aggressive post-transplant antibody monitoring/treatment strategy.

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