Abstract Background Acute kidney injury (AKI) in hospital-admitted patients is a common complication associated with increased mortality. The diagnosis of AKI relies on the ascertainment of peak increase in serum creatinine (SCr). This study evaluated the incidence of AKI using the increase from mean 7–365 days pre-admission (AKIpre) and admission (AKIadm) SCr levels, and examined the associations of AKI and changes in SCr levels with all-cause mortality. Methods A total of 2436 patients admitted to a tertiary hospital were recruited and followed-up for a median of 47.70 (interquartile range 18.20) months. AKI incidence and severity were defined according to the Kidney Disease: Improving Global Outcomes-AKI Guidelines. Follow-up data were collected from the Hospital Episode Statistics and Office of National Statistics. Mortality was evaluated during a short- (30 days), mid- (1 year) and long-term (4 years) period. Results No difference in the AKI rates using AKIpre and AKIadm (12.5% versus 12.2%; P = 0.695) or in the AKI severity (P = 0.261) was evident. Agreement between the two definitions was modest (Kappa-statistic = 0.596, P < 0.001). Patients with AKIpre or AKIadm had increased all-cause mortality compared with those without AKI during all follow-up periods. In fully adjusted regression analysis, AKIpre [hazard ratio (HR) = 2.226, 95% confidence interval (CI) 1.140–4.347; P = 0.027] and AKIadm (HR = 2.105, 95% CI 1.090–4.064; P = 0.027) remained associated with 30-day mortality. Results for the 1- and 4-year periods were similar. Increases of >4.00 μmol/L and >6.06% from pre-admission or >6.00 μmol/L and >17.24% from admission SCr levels presented increased mortality risk during follow-up. Conclusions Use of admission or pre-admission SCr provides similar incidence rates, but they diagnose different sets of patients. Even minor increases in SCr, below those required for the classification of AKI, were associated with increased mortality. These findings can help the clinicians to identify patients at higher risk for adverse outcomes.