Objectives: Sarcopenia is a well-known risk factor for inferior cancer outcomes, but the identification of patients at risk remains challenging. A new sarcopenia index (SI), defined as serum creatinine (Cr) x cystatin C (CysC)-based glomerular filtration rate (eGFRCysC), has been reported to be an objective surrogate marker for sarcopenia. The aim of this study was to assess whether the SI is associated with sarcopenia and cancer-related fatigue (CRF) in patients with advanced cancer. Methods: This cross-sectional study included 182 patients with different types of cancer (cancer stages III/IV; mean age 55.1 +/- 11.1 y). Sarcopenia was defined as the presence of both low muscle mass and low muscle strength. The cross-sectional area of skeletal muscle mass (SMA) at the third lumbar spine was estimated by computed tomography (CT). Low muscle mass was defined as a skeletal muscle index (SMA/height2) <34.9 cm(2)/m(2) for women and 40.8 cm(2)/m(2) for men. Low muscle strength was determined by handgrip strength (HGS) according to the cutoffs of the Asian Working Group for Sarcopenia (<18 kg for women and <26 kg for men). CRF was measured by the Brief Fatigue Inventory (BFI). The associations between SI with both sarcopenia and CRF were investigated. Results: The prevalence of sarcopenia was 27.5%. The SI was significantly lower in both the sarcopenia and severe fatigue groups. The associations between SI and SMA (r = 0.365; P < 0.001), skeletal muscle index (SMI) (r = 0.340; P < 0.001), and HGS (r = 0.414; P <0.001) were stronger than the associations between the serum creatinine/cystatin C (Cr/CysC) ratio and SMA (r = 0.299; P < 0.001), SMI (r = 0.269; P <0.001), and HGS (r = 0.364; P <0.001). Additionally, a decrease in the SI was associated with a higher likelihood of sarcopenia (odds ratio per 10-unit, 1.09; 95% confidence interval, 1.02-1.16) after adjusting for potential confounding factors. However, there was only a weak correlation between the SI and BFI score (r = -0.161, P = 0.045). Conclusion: The SI might be a useful objective tool for assessing sarcopenia in patients with advanced cancer. Further studies are warranted to extend the present findings. (C) 2020 Elsevier Inc. All rights reserved.