Afternoon Urine Osmolality Is Equivalent To 24-h In Healthy 3-13 y Children

Abstract

While daily hydration is best assessed in a 24-h urine sample, using a urine spot sample can be more practical for healthcare professionals, researchers, and individuals. Although urine product is subject to circadian variation, 24-h urine concentration reported to be approximated from a mid- to late-afternoon spot urine sample in adults. However, no data exists in children. PURPOSE: To identify time windows during which spot values of urine osmolality (UOsm) is representative of 24-h values in healthy children. METHODS: Among 541 healthy children (age: 3-13 y, female: 45%, BMI: 17.7±4.0 kg∙m-2), equivalent test was performed by comparing UOsm from specific time windows [morning (0600-1159), early afternoon (1200-1559), late afternoon (1600-1959), evening (2000-2359), overnight (2400-0559), and first morning (0600-0959)] to 24-h urine sample. The equivalency was determined when the mean difference and the confident interval between the spot and 24-h urine sample fell below the bound of 80 mmol·kg-1. The analysis was performed by using the first spot urine sample from each time window. Other spot urine samples after the first spot urine within each time window were not used to avoid unequally weighting data. RESULTS: Equivalence test showed that the late afternoon (1600-1959) spot urine sample UOsm value was equivalent to the 24-h UOsm value in children (P<0.05; mean difference: 62; CI: 45-78). The overall diagnostic ability of urine osmolality assessed at late afternoon (1600-1959) to diagnose elevated urine osmolality (>800 mmol·kg-1) on the 24-h sample was “good” (area under the curve: 87.4%; sensitivity: 72.6%; specificity: 90.5%; threshold: 814 mmol·kg-1). CONCLUSIONS: These data suggest that in free-living healthy children, 24-h urine concentration can be approximated from a late afternoon spot urine sample.