Acute kidney injury (AKI) is a heterogeneous clinical syndrome involving a range of risk factors and acute injuries that affect both short- and long-term clinical outcomes [1]. Studies have shown that AKI occurs in 10-15% of hospitalized patients and in over 50% of intensive care unit (ICU) patients [2,3]. Obesity has become an epidemic over recent years. Existing literature shows that AKI is common in surgical patients with severe obesity. Obesity is a new risk factor for AKI and a higher Body mass index (BMI) is associated with a higher incidence of AKI [4]. Obesity can also increase the risk of cardiovascular disease, diabetes and hypertension [5]. However, hospitalized and chronically ill patients show a J-shaped relationship between BMI and mortality and overweight and moderate obesity is associated with lower mortality; this represents an inverse epidemiological phenomenon known as the "obesity paradox"[6]. Studies have shown that the "obesity paradox" also exists in AKI and chronic kidney disease [7,8].

The most popular anthropometric measure for defining obesity is body mass index (BMI). The distribution and severity of obesity, however, may not be accurately determined by BMI [9]. Body surface area (BSA) is a measure to quantify body size that is frequently used to standardize other physiological measures (e.g., renal clearance, cardiac output), as well as a biometric unit for determining appropriate drug doses in cancer chemotherapy [10]. Numerous studies have demonstrated that BSA has a higher sensitivity than BMI as a predictive factor for numerous illnesses [11,12]. Verbraecken et al. [13] speculated that BSA is connected to variations in body fat distribution by reporting that BSA reflects additional dimensions than BMI and is not merely a body mass index. Studies on BSA and prognosis in critically ill patients with AKI are scarce, nevertheless. Therefore, this study was designed to investigate the correlation between BSA and 90-day all-cause mortality in critically ill patients with AKI.