Analyte variations in consecutive 24-hour urine collections in children

Ellison, J. S.; Hollingsworth, J. M.; Langman, C. B.; Asplin, J. R.; Schwaderer, A. L.; Yan, P.; Bierlein, M.; Barraza, M. A.; Defoor, W. R.; Figueroa, T. E.; Jackson, E. C.; Jayanthi, V. R.; Johnson, E. K.; Joseph, D. B.; Shnorhavorian, M.

J Pediatr Urol. 2017 Jul 26; 13(6):632.e1-632.e7

Abstract

PURPOSE: The metabolic evaluation of children with nephrolithiasis begins with a 24-h urine collection. For adults, the diagnostic yield increases with consecutive collections; however, little is known regarding the variability of multiple 24-h studies in the pediatric population. We sought to evaluate the variability of consecutive 24-h urine collection in children through a multi-institutional study hypothesizing that compared with a single collection, consecutive 24-h urine collections would reveal a greater degree of clinically useful information in the evaluation of children at risk for nephrolithiasis. MATERIALS AND METHODS: Including data from six institutions, we identified children less than 18 years of age considered at risk for recurrent nephrolithiasis, undergoing metabolic evaluation. We evaluated a subset of patients performing two collections with urine creatinine varying by 10% or less during a 7-day period. Discordance between repeat collections based on normative urine chemistry values was evaluated. RESULTS: A total of 733 children met inclusion criteria, and in over a third both urine calcium and urine volume differed by 30% or more between samples. Urine oxalate demonstrated greater variation between collections in children <5 years than among older children (p = 0.030) while variation in other parameters did not differ by age. Discordance between repeat samples based on normative values was most common for urine oxalate (22.5%) and the derived relative supersaturation ratios for both calcium phosphate (25.1%) and calcium oxalate (20.5%). The proportion of discordant samples, based on normative thresholds, as well as variability greater >/=30% and 50%, respectively, are shown in the table. CONCLUSIONS: Our analysis indicates that stone risk in as many as one in four children may be misclassified if normative values of only a single 24-h urine are used. In light of these findings, repeat 24-h urine collections prior to targeted intervention to modify stone risk are advised to increase diagnostic yield in children at risk for nephrolithiasis.

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