Objective：To investigate the value of neutrophil gelatinase - associated lipocalin（NGAL）and reticulocyte percentage （Ret%）in the differential diagnosis of acute kidney injury（AKI）and chronic kidney disease（CKD）. Methods：A total of 437 patients with nephropathy admitted to the Department of Nephrology of the First Affiliated Hospital with Nanjing Medical University from July 2016 to January 2020 were included. In this retrospective cohort study，patients were divided into AKI group（n=140）and CKD group （n=297）. After adjusting for intergroup confounding factors using propensity score matching（PSM），the differences in urinary NGAL/ serum NGAL（u/sNGAL）and Ret% were compared between the two groups. After stratifying renal function based on glomerular filtration rate（GFR），the efficacy of u/sNGAL，Ret%，and their product（u/sNGAL × Ret%）in differentiating AKI from CKD was evaluated. Results：The level of u/sNGAL ×Ret% in AKI group was significantly higher than that in CKD group[1.74（0.81，4.17）vs. 0.28（0.15，0.55），P ＜ 0.001]. Propensity-matched analysis included 46 patients in each group. After 1∶1 propensity score matching （46 cases in each group），the Ret%，u/sNGAL，and u/sNGAL×Ret% in the matched AKI group were significantly higher than those in the matched CKD group[1.75（1.26，2.53）vs. 1.37（1.16，1.83），P=0.027；0.64（0.33，1.52）vs. 0.31（0.13，0.76），P=0.006；and 1.27 （0.59，3.31）vs. 0.46（0.25，1.53），P=0.001]. In the entire patient population，the receiver operating characteristic（ROC）curve analysis indicated that Ret%，u/sNGAL and u/sNGAL×Ret% were all effective in differentiating AKI from CKD，with areas under the curve（AUC）of 0.701，0.848 and 0.870，respectively. The subgroup analysis stratified by renal function showed that in all subgroups， u/sNGAL×Ret% was significantly elevated in the AKI group（P ＜ 0.01）. In the subgroup analysis of patients with eGFR ≥ 60 mL/（min· 1.73 m² ），both u/sNGAL×Ret% and Ret% were effective in differentiating AKI from CKD. However ，in the subgroup analysis of patients with eGFR ＜ 60 mL/（min·1.73 m² ），u/sNGAL×Ret% was significantly better than Ret% in differentiating AKI from CKD（P ＜ 0.05）. Conclusion：u/sNGAL ×Ret% shows a good value in the differential diagnosis of AKI and CKD，especially in patients with impaired renal function，and could be used as a reliable index for clinical diagnosis.