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

<p><b><i>Background:</i></b> 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<sub>creat</sub>) and cystatin C (eGFR<sub>cystC</sub>) blood concentrations. <b><i>Methods:</i></b> 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. <b><i>Results:</i></b> Correlation between eGFR based on creatinine and cystatin C concentrations was <i>r</i> = 0.40 (<i>p</i> value <0.01). Higher BMI was associated with lower eGFR<sub>cystC</sub> but not with eGFR<sub>creat</sub>. Higher BSA was associated with higher eGFR<sub>creat</sub> and lower eGFR<sub>cystC</sub> (<i>p</i> value <0.05). Lean and fat mass percentages were associated with eGFR<sub>creat</sub> but not with eGFR<sub>cystC</sub>. <b><i>Conclusion:</i></b> Our findings suggest that both eGFR<sub>creat</sub> and eGFR<sub>cystC</sub> are influenced by BMI and BSA. eGFR<sub>creat</sub> is more strongly influenced by body composition than eGFR<sub>cystC</sub>.</p>