Urine Ammonium During Normothermic Machine Perfusion as a Marker of Donor Kidney Function

Background. Optimizing graft selection remains a central priority, and normothermic machine perfusion (NMP) provides a potential platform for pretransplant organ assessment. Kidney ammonium excretion quantifies proximal tubular and interstitial function. This study investigated urine ammonium during NMP as a marker of kidney graft function. Methods. Ammonium concentrations were measured in both perfusate and urine during 4 h of NMP in porcine kidneys exposed to controlled warm ischemia and in discarded human donor kidneys. Using a porcine autotransplant model, NMP urine ammonium was examined in relation to posttransplant outcomes. Results. Urine ammonium during NMP was reduced in ischemic porcine kidneys. At 4 h of NMP, higher urine ammonium concentration correlated with improved posttransplant kidney function, while other NMP parameters showed weaker or no correlation. In discarded human kidneys, NMP urine ammonium was highly variable and independent of kidney donor profile index, glomerulosclerosis, donor type, age, or preservation technique, indicating potential additive value beyond current assessment criteria. The findings remained robust after adjustment for urine flow, urine creatinine, and perfusate pH. Conclusions. Urine ammonium during NMP may provide additional functional information about kidney graft quality. Its association with posttransplant function supports further investigation as a potential adjunct to current assessment criteria.