The typical distribution of urine CyC has been described for healthy adults and those with AKI in nontransplant settings [9
]. The present study is important as the first to demonstrate: (1) the distribution of urine CyC in recipients immediately following kidney transplantation relative to current definitions of graft function, (2) the utility of peritransplant urine CyC for predicting DGF and (3) the association of peritransplant urine CyC levels with 3-month allograft function.
We noted a stepwise difference in urine CyC and urine CyC/urine Cr levels according to graft function with minimal overlap. Percent change in urine CyC from 0 h to each subsequent time-point was also clearly different for those at opposite ends of the spectrum for graft function, but there was substantial overlap between SGF and both DGF and IGF at various time-points. This overlap likely explains the marginal diagnostic performance of urine CyC for DGF.
Urinary levels of CyC increase because of its high filtered load with subsequent abnormal reabsorption due to proximal tubular cell dysfunction or injury. Concentrations return to lower levels as the filtered load and tubular function/reabsorption improves. The acute drop in urine CyC for those with IGF in the current study supports this idea and suggests that despite variable periods of complete ischemia, a subset of kidneys will continue to have well-functioning tubules once reperfusion takes place. Alternatively, release of preformed CyC into the urine by injured/leaking tubular cells could contribute to urinary CyC levels in AKI.
Importantly, urine CyC levels here are on a par with those seen in a nontransplant setting of severe acute tubular necrosis. Herget-Rosenthal et al. [15
] reported median urine CyC was 1.7 g/mol Cr (15 mg/g in terms of urine CyC/urine Cr) for 26 patients with acute tubular necrosis requiring dialysis support versus 0.1 g/mol Cr (0.8 mg/g) for 47 patients with acute tubular necrosis who did not require dialysis. These numbers are within 1 SD of mean urine CyC/urine Cr on the second POD in patients with either DGF or IGF.
We presented both absolute urine CyC concentrations and urine CyC/urine Cr. We and others have previously reported no significant differences after dividing by urine Cr [1
], though most of this data remains unpublished. It is important to note there are no clear guidelines for performing this ‘adjustment’ in the non-steady state. Urine Cr may only modestly ‘correct’ for urine output/flow following IRI with the development of isosthenuria, while the amount of any urinary molecule may appear higher after dividing by low urine Cr due to its impaired excretion [17
]. Thus, dividing by urine Cr could either hinder or erroneously improve the performance of urinary biomarkers. A timed urine collection for total biomarker excretion may be the most accurate measure, but it is too impractical and error-prone to be used clinically. Large multicenter trials are needed to clarify the role of urine Cr for biomarker development in various clinical settings, including transplantation.
Several studies have presented data on a variety of novel urinary biomarkers’ ability to detect AKI prior to serum Cr [19
]. Results of the current study build on our earlier data [1
]. While we previously reported a lack of utility for kidney injury molecule-1 and good utility for neutrophil gelatinase-associated lipocalin and IL-18, our current findings demonstrate urine CyC, a biomarker of proximal tubular function, has moderate utility for predicting DGF, with the ratio of urine CyC/urine Cr at 6 h yielding the best AUC.
Given our previous results showing an association between biomarkers of structural kidney injury at the time of transplant and both early and later allograft function, it was interesting to see similar findings with a urine biomarker of proximal tubular function. Our limited sample size did not allow us to combine biomarkers of structural kidney injury (neutrophil gelatinase-associated lipocalin and IL-18) and tubular function (CyC) to determine whether more robust associations with early and later graft function are possible. Evaluating biomarkers of structural injury, which may be too sensitive in the transplant setting, along with biomarkers of tubular function may advance our assessment of meaningful allograft injury in the peritransplant period. Based on the growing body of evidence linking AKI and chronic kidney disease progression [24
], understanding the relationship between the degree of structural kidney injury and tubular dysfunction or ‘stunning’ may be a crucial step toward improving patient outcomes in nephrology.
We have previously described the strengths and limitations of this observational study [1
]. Given our fairly small sample size, larger studies should be conducted in the transplant setting to verify our findings. Additional considerations include the inconsistency of protocols for measuring/recording urine output between centers. Future studies should standardize this practice and attempt to measure baseline urine output (residual function) to help clarify its diagnostic/prognostic role and better address the need for urine Cr ‘adjustment’ and the role that native kidney Cr excretion plays in this unique setting. The impact of pretransplant dialysis on the filtered load for both CyC and Cr needs further study with serum and urine values obtained just prior to transplant. Furthermore, the effect of baseline GFR, and other factors, on biomarker performance needs clarification in larger multicenter trials in diverse settings of AKI.
In summary, we have shown that urine CyC is an early and moderately accurate predictor of DGF. Future studies are needed to validate our findings, maximize our understanding of CyC and other novel biomarkers in this population, and account for additional donor/recipient variables that may impact biomarker values (donor renal function and AKI, immunosupression regimens, etc.). The growing body of literature surrounding biomarkers in transplantation speaks to the need to include them in any future interventional trials for DGF [25
]. Urine CyC and other biomarker concentrations in the immediate post-transplant setting may provide valuable insight into the physiology and early diagnosis of IRI, or even describe the ‘biomarker fingerprint’ of renal recovery following severe AKI.