Objective:To investigate the effect of serum cystatin C level on the occurrence and its long-term prognosis of contrast agent-induced acute kidney injury(CI-AKI)after percutaneous coronary intervention(PCI)in elderly patients.Methods:A total of 848 elderly patients(≥60 years)undergoing PCI in our department between Mar 2015 and Dec 2017 were enrolled in a prospective cohort.The CI-AKI was defined as the increase of serum creatinine ≥44.2 μmol/L within 48-72 h after using iodine contrast agent or more than 25 % higher than base level within 48-72 h after PCI.A receiver operating characteristic curve was used to analyze the optimal cut-off value of Cystatin C for predicting CI-AKI after PCI.Patients were divided into 2 groups based on the optimal cut-off value of Cystatin C: the high Cystatin C group(Cystatin C ≥1.3 mg/L, n=178)and the control group(Cystatin C<1.3 mg/L, n=670). The differences in the incidence of CI-AKI after PCI and major adverse cardiac events(MACE)at 1 year follow-up were compared between the two groups.The Cox regression model was further used to analyze the predictors of the long-term prognosis after PCI.Results:Of 848 patients receiving PCI, the incidence of CI-AKI was 9.4%.The incidence of MACE at 1 year after PCI was higher in the high Cystatin C group than in the control group(15.7% vs.9.3%, χ2=6.524, P=0.011). Cox regression analysis confirmed that the high baseline level of Cystatin C was the most independent predictive factor for MACE at 1 year of follow-up( HR=16.244, P<0.001). Conclusions:The high baseline level of Cystatin C(≥1.3 mg/L)is an independent risk factor for CI-AKI and is also the most important predictor for the occurrence of long-term MACE in elderly patients undergoing PCI.