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

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 Tc-99 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.