Supplementary Material for: Childhood Estimates of Glomerular Filtration Rate Based on Creatinine and Cystatin C: Importance of Body Composition

Background: Creatinine and cystatin C concentrations are commonly used to estimate glomerular filtration rate (eGFR) in clinical practice and epidemiological studies. To estimate the influence of different body composition measures on eGFR from creatinine and cystatin C blood concentrations, we compared the associations of different anthropometric and body composition measures with eGFR derived from creatinine (eGFR_creat) and cystatin C (eGFR_cystC) blood concentrations. Methods: In a population-based cohort study among 4,305 children aged 6.0 years (95% range 5.7-8.0), we measured weight and height and calculated body mass index (BMI) and body surface area (BSA), and lean and fat mass using dual-energy X-ray absorptiometry. At the same age, we measured creatinine and cystatin C blood concentrations and estimated the GFR. Results: Correlation between eGFR based on creatinine and cystatin C concentrations was r = 0.40 (p value <0.01). Higher BMI was associated with lower eGFR_cystC but not with eGFR_creat. Higher BSA was associated with higher eGFR_creat and lower eGFR_cystC (p value <0.05). Lean and fat mass percentages were associated with eGFR_creat but not with eGFR_cystC. Conclusion: Our findings suggest that both eGFR_creat and eGFR_cystC are influenced by BMI and BSA. eGFR_creat is more strongly influenced by body composition than eGFR_cystC.