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Long-term Outcomes of Postoperative Kidney Injury: Comment.

To the Editor: IN recent article by Turan et al., we noted several issues that were not well addressed. First, according to the Kidney Disease Improving Global Outcomes program, a criterion of serum creatinine greater than1.5 mg/dl or estimated glomerular filtration rate less than 60 ml · min · 1.73m used in this study only excluded moderate and severe chronic kidney disease (stages 3 to 5), but not occult or mild chronic kidney disease (stages 1 and 2), which has been identified as an independent risk factor for postoperative acute kidney injury (AKI) and progressive chronic kidney disease. Furthermore, it was unclear why preoperative serum creatinine and estimated glomerular filtration rate were not included in baseline characteristics of patients and compared between patients with and without AKI. Thus, this study cannot exclude the possibility that preoperative occult or mild renal dysfunction had been attributable to postoperative long-term renal outcomes. Second, this study only assessed the effects of AKI severity on the long-term adverse renal outcomes but did not take contribution of AKI duration to them. Actually, severity and duration of postoperative AKI are the important factors affecting long-term adverse renal outcomes including chronic kidney disease development, regardless of pre-existing renal function. The severer and longer postoperative AKI are, the more frequent long-term adverse renal outcomes occur. Most importantly, even after early recovery, postoperative AKI is also associated with an increased risk of chronic kidney disease in a graded manner related to AKI severity and persistence. Furthermore, persistent AKI is a main determinant for progression of chronic kidney disease. We argue that not taking contribution of AKI duration may be one reason why long-term renal prognosis was comparable among patients with a different stage of AKI in this study. Third, the multivariable model assessing associations of postoperative AKI severity with long-term renal prognosis and mortality only included baseline and intraoperative data for adjustments but not postoperative data. Other than basic illness conditions and surgical burdens, long-term renal prognosis and mortality of patients with postoperative AKI are also dependent on changes of illness conditions and the occurrence of postoperative adverse events. It is well believed that postoperative cardiac dysfunction, sepsis, new-onset proteinuria, and use nephrotoxic drugs are significantly attributable to transition of postoperative AKI to chronic kidney disease. Furthermore, postoperative pulmonary and neurologic complications, heart failure, and new-onset arrhythmia are the known precipitating factors for shortand long-term mortality after noncardiac surgery. Because of this limitation, we are concerned that statistical adjustment for hazard ratios of different stages of AKI for long-term adverse renal outcomes and mortality would have been biased.