Previous studies have reported that the renal recovery rate after AKI is between 33% and 90% . In this study, the overall renal recovery rate of all patients was 49.4%. The difference in renal recovery was related to the different recruited cohorts, diagnostic criteria for AKI, definition of renal recovery and follow-up time. At present, there is no unified consensus on the definition of renal recovery and the duration of follow-up. Long et al.  used serum creatinine as the research index to contrast the effect of the definition of four different degrees of renal recovery on the progression to CKD and survival. Their study showed that the optimal definition of renal recovery to predict survival was a reduction in blood creatinine to within 1.5 times baseline within 30 days. Patients whose serum creatinine decreased to 1.5 baseline had significantly better 1-year survival than patients with no renal recovery within 30 days; the risk of developing and progressing to CKD within 5 years was significantly increased if the patient's renal recovery was limited to serum creatinine falling to 1.25–1.5 times the baseline within 30 days. Their study served as an important reference for our definition of renal recovery criteria and the duration of follow-up.
Previous studies have suggested that advanced age is an unfavorable factor for renal recovery [8, 9]. To elaborate on the characteristics of renal recovery in elderly and nonelderly cohorts, we used the Youden index to find the cutoff value of age affecting renal recovery of 63 years, which was used to divide all patients into two cohorts, elderly and nonelderly. These two cohorts were analyzed separately for risk factors influencing renal nonrecovery after AKI, which had not been reported in previous studies. We found that AKI stage and hemoglobin were common risk factors in both cohorts. Forni et al.  reviewed previous studies and found that the AKI stage was negatively correlated with renal recovery, and the higher the stage of AKI was, the lower the rate of renal recovery. A total of 288 patients had renal recovery in this study, including 51.0% of patients with stage 1 AKI, 35.1% of patients with stage 2 AKI, and 13.9% of patients with stage 3 AKI, which suggested that patients with higher AKI stages had a lower rate of renal recovery. Dividing all renal recovery patients into two groups with 63 years as the cutoff point, a lower percentage of patients with higher AKI stages could still be found in both groups, with 16.7% of AKI stage 3 patients in the age < 63 years cohort and 8.2% of AKI stage 3 patients in the age ≥ 63 years cohort. Few studies have evaluated the role of anemia in renal recovery after AKI. Our study found that among all patients, the level of hemoglobin was a risk factor associated with renal nonrecovery. In a retrospective single-center study  that enrolled 41 patients with AKI requiring renal replacement therapy, anemia was associated with renal recovery at discharge. Another retrospective study  that included 211 patients with AKI suggested that the effects of anemia may be more pronounced in severe cases of AKI, in which case the effects of prolonged hypoxia may interfere with the extent and rate of renal recovery.
The APACHE II score and SOFA score are currently in widespread clinical use to predict outcomes in critically ill patients. In this study, we used these two scores as possible risk factors associated with renal nonrecovery, and we found that these two scores in the age < 63 years cohort were significantly different in univariate analysis, but neither score was a risk factor for renal nonrecovery in multivariate analysis. However, it was different in the age ≥ 63 years cohort, and the APACHE II score was an independent risk factor for renal nonrecovery. Mehta et al.  suggested that higher APACHE III scores were associated with a lower rate of renal recovery in acute renal failure patients. A meta-analysis  that included 14 studies with 4405 patients with AKI found that SOFA scores were associated with renal recovery among patients who required continuous renal replacement therapy for AKI. This illustrated that older patients aged ≥ 63 years may have lower rates of renal recovery with greater disease severity.
Past medical history was also considered an important factor associated with renal recovery, and the most common factors were hypertension, diabetes mellitus, and coronary heart disease . Univariate analysis in this study found that diabetes mellitus was a risk factor for renal nonrecovery in all patients, but multivariate analysis was performed and showed that diabetes mellitus was independently associated with renal recovery only in the elderly cohort. This suggested that diabetes mellitus caused more damage than other chronic diseases. Some studies  suggested that once diabetes mellitus occurs, most patients continue to manifest progressive renal damage even if glycemia is tightly controlled. This suggested that key pathogenic mechanisms involved in the induction and progression of diabetic nephropathy remain active, and no effective treatment is currently available. Studies have suggested that the reason diabetes mellitus emerges as an independent risk factor among patients of advanced age is because there was a positive correlation between the damage to the kidneys caused by diabetes mellitus and the length of the disease course .
Serum creatinine has been widely used for the evaluation of renal recovery after AKI, and most studies chose to use the creatinine value before AKI as the baseline; however, individual studies varied in their choice of time point before AKI considered to be the baseline. This study used the creatinine value at the time of AKI diagnosis as the observation index. Univariate analysis found lower creatinine values in the renal recovery group than in the no renal recovery group among all patients, and multivariate analysis found that creatinine values were a predictor of renal recovery after AKI only in the age ≥ 63 years cohort.
It is well known that surgery under general anesthesia increases the burden on renal function, but we do not know whether undergoing surgery under general anesthesia before AKI affects renal recovery. Our study found that among all patients, the proportion of surgery in the group without renal recovery was significantly higher than that in the renal recovery group, but multivariate analysis suggested that surgery under general anesthesia was associated with renal recovery only in patients aged ≥ 63 years, which may be related to decreased organ function reserve leading to decreased tolerance to surgery under general anesthesia in elderly patients.
There are certain limitations of this study. First, we included more independent variables, but our sample size was relatively small after grouping, although it basically satisfied the requirement of ten times the number of independent variables. Second, this was a retrospective observational study, which may have generated selection bias, although we included patients from multiple centers and employed multivariate regression analysis to correct for confounders. Third, we had a shorter follow-up of our patients, and there are studies that considered a follow-up of three months to be more appropriate. These factors all may generate bias, which still needs further validation by large-scale prospective studies in the future.