*Purpose: Thrombotic microangiopathy (TMA) is a dangerous disorder characterized by fragmented red blood cells, decreased platelet count, and organ failure due to thrombosis. We investigated the prevalence, diagnosis, treatment, and long-term outcome of kidney transplant-associated TMA (KTA-TMA) retrospectively.

*Methods: We conducted a retrospective study of kidney transplantations performed between January 1999 and November 2019 at our institution. During the study period, 1108 renal transplantations were performed, and 37 patients were diagnosed as having TMA by pathological findings. In those cases, two cases were excluded because of STEC-HUS and TTP patients, and other nine cases were also excluded because of not significant from clinical course (a. TMA finding was obtained from Zero-hour biopsy, b. obtained from protocol biopsy with stable renal function and/or only one glomeruli finding). In addition to these other 26 cases, two case was diagnosed as Posttransplant aHUS by clinical course without pathological finding, total 28 recipients (25 living and 3 decreased) were diagnosed as having KTA-TMA. During the study period, total 284 ABO-incompatible transplantations were performed.

*Results: 28 recipients were diagnosed as having KTA-TMA. No significant differences were found in sex and living or deceased transplantation. The prevalence of ABO incompatible cases (15 cases) with KTA-TMA was higher than that of ABO compatible cases (13 cases; p< 0.01). TMA developed within 1 month after transplantation in almost all the cases. Genetic tests were performed in 3 cases, and all had genetic abnormalities. Almost all cases had decreased platelet count and hemoglobin level and increased lactate dehydrogenase levels, but no relationship was found with the timing of these changes. Using our previously proposed diagnostic scoring system of TMA, 17 cases were diagnosed as KTA-TMA (60.7%). At this time, we proposed newly altered diagnostic criteria, which the item ‘Platelet Count reduction’ has been added, 27 cases were diagnosed as KTA-TMA (96.4%). The positivity rate without TMA was 0%. As treatments, plasma exchange therapies were performed in almost all the cases. In 7cases, eculizumab infusion was given. We confirmed the disappearance of TMA in 16 (57.1%) of the 28 cases via biopsy examination. Except 3 cases for which hemodialysis therapy was restarted, all cases showed good graft survival.

*Conclusions: ABO incompatibility is a significant risk factor of KTA-TMA, and prompt diagnosis using the Rapid KTA-TMA Diagnostic Criteria and immediate treatment may lead to good long-term outcomes

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