Body mass does not have a clinically relevant effect on cystatin C eGFR in children

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Abstract
Background. Unlike creatinine, Cystatin C (CysC) is believed to be independent of body composition in both adults and children. Recent findings in adults, suggesting an improved performance of CysC-based estimated glomerular filtration rate (CysC eGFR) by accounting for body mass, necessitated a careful re-evaluation of this issue in children. Methods. We studied 240 children (median age 11.7 years, range 2–17.9 years, 107 girls), with various kidney diseases, for any change in the relationship between 99 Tc DTPA GFR and CysC eGFR after accounting for body mass. For body mass assessment, body mass index (BMI) z -score was calculated using height-adjusted age, to account for growth retardation secondary to chronic kidney disease. Results. CysC eGFR did not have a significant correlation with BMI z -score (correlation coefficient = 0.06; P = 0.34). Accounting for BMI z -score did not add to the 65% variance in nuclear GFR explained by CysC eGFR. Moreover, it did not change the regression coefficient of 0.85 between CysC eGFR and nuclear GFR either. On Bland & Altman analysis, the bias of 0.05 and standard deviation of 20.39 also did not improve after accounting for BMI z -score in the revised CysC eGFR formula. Conclusions. In children, body mass exerts a minimal effect on the performance of CysC eGFR estimation.