BACKGROUND: Acute kidney injury (AKI) after cardiac surgery is a common complication associated with increased mortality. However, the heterogeneity of the definitions used results in high variance of incidence rates in the literature. Data on the effect of diabetes mellitus on AKI incidence in this setting are scarce. We thus aimed to compare the incidence of AKI (defined by the AKIN, RIFLE and KDIGO criteria) in diabetic vs. non-diabetic patients undergoing cardiac surgery.METHODS: This is a nested case-control study from a cohort of patients undergoing cardiac surgery between 1/1/2013 and 30/6/2014 in a single center. Exclusion criteria were: type-1 diabetes, end-stage renal disease, death during surgery and AKI prior to surgery. We identified 199 type-2 diabetic patients and matched them for gender, age and estimated glomerular filtration rate (eGFR) to 199 non-diabetic individuals. The incidence of AKI between the two groups was compared in the total population and in subgroups according to preoperative eGFR. Univariate and multivariate logistic regression analysis were conducted to identify factors associated with AKI.RESULTS: The incidence of AKI was moderately high, but similar between the two study groups (AKIN and KDIGO: 24.1 vs. 23.1 %; p = 0.906, RIFLE: 25.1 vs. 25,1 %; p = 1.000, in diabetics and non-diabetics respectively). A trend towards increased incidence of AKI from eGFR subgroup 1 to subgroup 3a was noted in diabetic patients (p = 0.04). No significant differences were detected between the two study groups within any eGFR subgroup studied. At multivariate analysis, age [per year increase: odds ratio (OR) 1.034, 95 % confidence interval (CI) 1.001-1.068] and duration of cardiopulmonary bypass [per minute increase: OR 1.009 (1.003-1.015)] were associated with AKI. Diabetes was not related to AKI development in regression analysis [OR 1.057 (0.666-1.679)].CONCLUSIONS: Incidence of AKI after cardiac surgery is high, but diabetes is not a risk factor for AKI. Baseline renal function in diabetics is related inversely to the incidence of AKI. Age and cardiopulmonary bypass duration are independent predictors of cardiac surgery-associated AKI.