Assessment of Within- and Inter-Patient Variability of Uremic Toxin Concentrations in Children with CKD.

To promote improved trial design in upcoming randomized clinical trials in childhood chronic kidney disease (CKD), insight in the within- and inter-patient variability of uremic toxins with its nutritional, treatment- and patient-related confounding factors is of utmost importance. In this study, the within- and inter-patient variability of a selection of uremic toxins in a longitudinal cohort of children diagnosed with CKD was assessed, using the intraclass correlation coefficient (ICC) and the within-patient coefficient of variation (CV). Subsequently, the contribution of anthropometry, estimated glomerular filtration rate (eGFR), dietary fiber and protein, and use of (prophylactic) antibiotics to uremic toxin variability was evaluated. Based on 403 observations from 62 children (median seven visits per patient; 9.4 ± 5.3 years; 68% males; eGFR 38.5 [23.1; 64.0] mL/min/1.73 m(2)) collected over a maximum of 2 years, we found that the within-patient variability is high for especially protein-bound uremic toxins (PBUTs) (ICC < 0.7; within-patient CV 37-67%). Moreover, eGFR was identified as a predominant contributor to the within- and inter-patient variability for the majority of solutes, while the impact of the child's anthropometry, fiber and protein intake, and antibiotics on the variability of uremic toxin concentrations was limited. Based on these findings, we would recommend future intervention studies that attempt to decrease uremic toxin levels to select a (non-dialysis) CKD study population with a narrow eGFR range. As the expected effect of the selected intervention should exceed the inter-patient variability of the selected uremic toxins, a narrow eGFR range might aid in improving the trial design.