['In our clinical experience, cystatin C (CysC) concentrations are not as high as expected in patients with chronic kidney disease (CKD) and high-stage renal dysfunction. We therefore investigated whether measurements of serum CysC result in an underestimation of renal dysfunction in pediatric patients with CKD.', 'Glomerular filtration rate (GFR) was estimated from serum creatinine (Cr) concentration, using the equation Cr-GFR (%)\xa0=\xa0[0.30\xa0×\xa0body length (m)/serum Cr]\xa0×\xa0100; and from serum CysC concentration, using the equation Cys-GFR (%)\xa0=\xa0(0.70/serum CysC)\xa0×\xa0100. We investigated the relationship between GFR estimated by these 2 equations. Patients aged 2-12\xa0years were assorted into 5 groups, based on GFR-Cr categories of <12.5, ≥12.5 to <25, ≥25 to <50, ≥50 to <75, and ≥75%, and GFR-CysC/GFR-Cr ratios were compared in these 5 groups.', 'The median GFR-CysC/GFR-Cr ratio in groups of patients with GFR-Cr of <12.5, ≥12.5 to <25, ≥25 to <50, ≥50 to <75, and ≥75% were 2.28, 1.48, 1.22, 1.18 and 0.98, respectively, with statistically significant differences between any two groups (p\xa0<\xa00.001).'] Do measurements of serum cystatin C concentrations underestimate renal dysfunction in pediatric patients with chronic kidney disease?