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Background: Creatinine-based estimated glomerular filtration rate (eGFR CRE ) may overestimate kidney function in patients with sarcopenia. While cystatin C-based eGFR (eGFR CYS ) is less affected by muscle mass, it may underestimate kidney function in patients with obesity. We sought to evaluate the relationship between body composition defined by computed tomography (CT) scans and discordance between creatinine, eGFR CRE and eGFR CYS in adult patients with cancer., Methods: This study is a cross-sectional study of consecutive adults with cancer with an abdominal CT scan performed within 90 days of simultaneous eGFR CRE and eGFR CYS measurements between May 2010 and January 2022. Muscle and adipose tissue cross-sectional areas were measured at the level of the third lumbar vertebral body using a validated deep-learning pipeline. CT-defined sarcopenia was defined using independent sex-specific cut-offs for skeletal muscle index (<39 cm 2 /m 2 for women and <55 cm 2 /m 2 for men). High adiposity was defined as the highest sex-specific quartile of the total (visceral plus subcutaneous) adiposity index in the cohort. The primary outcome was eGFR discordance, defined by eGFR CYS  > 30% lower than eGFR CRE ; the secondary outcome was eGFR CYS  > 50% lower than eGFR CRE . The odds of eGFR discordance were estimated using multivariable logistic regression modelling. Unadjusted spline regression was used to evaluate the relationship between skeletal muscle index and the difference between eGFR CYS and eGFR CRE ., Results: Of the 545 included patients (mean age 63 ± 14 years, 300 [55%] females, 440 [80.7%] non-Hispanic white), 320 (58.7%) met the criteria for CT-defined sarcopenia, and 136 (25%) had high adiposity. A total of 259 patients (48%) had >30% eGFR discordance, and 122 (22.4%) had >50% eGFR discordance. After adjustment for potential confounders, CT-defined sarcopenia and high adiposity were both associated with >30% eGFR discordance (adjusted odds ratio [aOR] 1.90, 95% confidence interval [CI] 1.12-3.24; aOR 2.01, 95% CI 1.15-3.52, respectively) and >50% eGFR discordance (aOR 2.34, 95% CI 1.21-4.51; aOR 2.23, 95% CI 1.19-4.17, respectively). A spline model demonstrated that as skeletal muscle index decreases, the predicted difference between eGFR CRE and eGFR CYS widens considerably., Conclusions: CT-defined sarcopenia and high adiposity are both independently associated with large eGFR discordance. Incorporating valuable information from body composition analysis derived from CT scans performed as a part of routine cancer care can impact the interpretation of GFR estimates., (© 2024 The Authors. Journal of Cachexia, Sarcopenia and Muscle published by Wiley Periodicals LLC.)