*Purpose: The aim of our study was to investigate biomarkers in a blood-based perfusate of normothermically perfused discarded human kidneys using urine recirculation as a volume and homeostasis control.

*Methods: Non-transplantable, discarded human kidneys were included in this study. Kidneys were perfused up to 24 hours. Hemodynamic and biochemical perfusion parameters were analyzed. Perfusate samples were collected at 5 time points (at 1, 6, 12, 18 and 24h). Neutrophil gelatinase-associated lipocalin (NGAL), kidney injury molecule-1 (KIM-1) and liver fatty acid-binding protein (L-FABP) levels in the perfusate were measured.

*Results: Twenty-four discarded human kidneys were perfused for 19±6.25 hours. Median CIT was 20.3 hours (10.4-108.1), median WIT was 15 min (9-33) for DCD kidneys, median urine output was 505 ml (10-11325) and median arterial flow was 330 ml/min (187-787). KIM-1 and L-FABP correlated significantly with length of CIT, but not with any of the perfusion parameters. There was no association between median arterial flow and urinary or serum KIM-1 levels. Median NGAL values were significantly lower in the group of kidneys with a median flow >330 ml/min; 29.4±6.2 ng/ml vs 73.2±15.9 ng/ml, p=0.0088. Delta NGAL values were significantly lower in the higher flow group; 5.9±1.6 ng/ml vs 21.9±7.2 ng/ml, p=0.019. The higher the total urine production, the higher the changes of NGAL, p=0.022.There was a correlation between arterial flow and lactate level.The longitudinal median arterial flow throughout the perfusion was higher in kidneys with a lactate level less than the median value of 14.2 mmol/l at the end of perfusion; p=0.016, 95% CI -251.9 – -40.1. The median arterial flow correlated significantly with the final lactate measurement (12.9±6.5 mmol/l); the higher the flow the lower the lactate; p=0.04, correlation coefficient (Spearman’s rho) = -0.444. Furthermore, the mean arterial flow (377.6±153 ml/min) correlated with the lactate level measured 6h after perfusion start (12.9±5.3 mmol/l); p=0.029, correlation coefficient (Spearman’s rho) = -0.475. The delta-value between the lactate at the beginning and after 6 hours of normothermic perfusion (0.77 ± 4.98 mmol/l) also correlated significantly with the median arterial flow; p=0.016, correlation coefficient (Spearman’s rho) = -0.519. Lactate levels did not correlate with the total or hourly urine output.

*Conclusions: Interpretation of parameters in isolated kidney perfusion is complex. In context with lactate levels, NGAL and arterial flow seem to be promising to judge kidney quality whilst on the perfusion circuit.

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