INTRODUCTION: Renal resistive index (RRI), derived from Doppler ultrasound sonography (DUS), is widely utilized for monitoring in kidney allotransplantation. However, its applicability in kidney xenotransplantation (KXTx) has not been thoroughly investigated. This study assesses the potential of RRI as an early predictive indicator of thrombotic microangiopathy (TMA), a common diagnosis, often associated with graft loss in KXTx. METHODS: Five consecutive pig-to-baboon KXTx were included in this prospective study. The first three cases comprised the study group, while the subsequent two served as a validation group. Serial RRI measurements were performed and compared with histopathological findings from biopsies. RESULTS: Baseline RRI values were significantly lower in the kidneys of naïve pigs compared to naïve baboons (median IQR: 0.597 0.568-0.603 vs 0.640 0.624-0.653, respectively; p < 0.02). In the study group, elevated RRIs were observed at the time when TMA was confirmed by biopsy (median IQR: 0.820 0.696-0.883 vs 0.628 0.575-0.717, respectively; p < 0.02). The provisional RRI cutoff value of 0.642 (ROC AUC = 0.857, p < 0.04) was predictive for TMA diagnosis, with a sensitivity of 1.000 and specificity of 0.667. In the validation group, RRI on POD 17 and 19 increased above 0.642, respectively, despite normal serum creatinine levels. Subsequent biopsies confirmed TMA on POD 21 and 22, respectively. CONCLUSION: RRI may be a sensitive method for the early detection of commonly diagnosed TMA in kidney xenografts, even when serum creatinine levels remain within normal ranges, indicating no apparent renal dysfunction. These results suggest that RRI may provide a valuable monitoring tool for KXTx.
Fujiwara et al. (2026) studied this question.