Renal dysfunction carries a substantial risk for cardiovascular morbidity and mortality, and the risk increases with a decline in kidney function [1]. Renal dysfunction is an independent predictor of mortality in stroke patients [2]. Most population-based studies have used estimated glomerular filtration rate based on serum creatinine (eGFRcreatinine) to evaluate renal function [3], [4]. Cystatin C (CysC) is a cysteine protease inhibitor that is freely filtered by the glomerulus and has been proposed as a more sensitive marker of renal function than serum creatinine or eGFRcreatinine because its serum concentrations are independent of muscle mass and do not seem to be affected by age or sex [5], [6]. It has been shown that serum CysC is superior in predicting mortality and cardiovascular events among elderly people and among elderly diabetic patients compared to serum creatinine or eGFRcreatinine [7]. Previous studies also have shown that CysC is a predictor for diabetes mellitus (DM) according to hemoglobin A1c (HbA1c) levels [8] and related to the incidence of DM over a 15-year follow-up period [9]. Ichikawa et al. [10] found an independent association between DM and incidence of brainstem infarction (BSI). In addition, elevated HbA1c levels predict the severity of stroke events and unfavorable outcomes in BSI patients [11].

Some studies have concluded that elevated serum CysC levels are associated with increased risk of ischemic stroke and predict cardiovascular events, early neurological deterioration, and death after ischemic stroke onset [12], [13]. However, many studies showed that this association was diminished and lost statistical significance after adjustment for traditional vascular risk factors [14], [15]. In both retrospective and prospective studies, regardless of the number of patients included, it is suggested that CysC may be associated with stroke progression, but for stroke prognosis, there is no correlation between serum creatinine-based eGFR and poor functional outcome of stroke, and eGFR based on CysC (eGFRCysC) is only associated with poor prognosis of stroke under certain conditions. Therefore, we wanted to explore under what conditions eGFRCysC is related to the prognosis of stroke, whether it is a special site or a special factor. The findings led us to wonder whether CysC might influence only a specific stroke subgroup, such as BSI patients. The predictive value of serum CysC for prognosis following BSI has not been elucidated. The objectives of this study were to determine whether the addition of eGFR (eGFRCysC) calculated with the use of the recently developed serum CysC equations would strengthen the relationships between various eGFR categories and the prediction of prognosis after BSI, as compared with the use of eGFRcreatinine. We hypothesized that serum CysC (eGFRCysC) could be an important predictor for the risk of prognosis during the six-month follow-up period after acute BSI.