Equivalence of Spot and Twenty Four Hour Urine Osmolality in Young Adults

Abstract

Urinary biomarkers of hydration can discriminate euhydration from hypohydration and are responsive to changes in daily total water intake. However, urine concentration is subject to daily fluctuation and circadian variation has been reported. Given the time and burden associated with 24 hour urine collections, establishing a time-frame during which spot urine concentration is equivalent to the 24 hour value would have utility for researchers, coaches, clinicians and individuals seeking to monitor hydration. PURPOSE: To determine the equivalence of multiple daytime spot urine osmolalities (UOsm_Spot) to the 24 hour sample (UOsm_24). METHODS: During three consecutive days, 82 adults (50% women; age 23.6 ± 2.9 years; BMI 22.2 ± 1.5 kg·m−2) recorded all ad libitum food and fluid intake and avoided physical activity that would result in substantial sweat loss. On the third day, subjects collected each void in a separate collection container over a full 24 hour period. The time of each void was recorded. UOsm was measured on each sample individually and on the 24 hour pool. Equivalence was set as ±100 mOsm·kg−1 from the 24 hour value, and was evaluated using the two one-sided test (TOST) method with α = 0.05. RESULTS: Mean total daily fluid intake (TFI) was 1830 ± 648 (range: 687 - 4350) ml·day−1, and mean UOsm_24 was 567 ± 214 (range: 149 - 1029) mOsm·kg−1. UOsm_Spot from samples produced between 14:00 and 20:00 were equivalent to UOsm_24, while UOsm_Spot between 10:00-12:00 and 12:00-14:00 was not equivalent. Samples between 10:00-12:00 and 12:00-14:00 tended to overestimate UOsm_24; the mean [95% CI] difference between UOsm_24 and UOsm_Spot was 83 [12;154] between 10:00-12:00; and 107 [58;157] between 12:00-14:00. From 14:00-16:00; 16:00-18:00; and 18:00-20:00, the mean [95% CI] differences between UOsm_24 and UOsm_Spot were −25 [−72;22]; 29 [−35;92]; and 12 [−41;66], respectively. CONCLUSION: Urine osmolality measured in voids produced between 14:00 and 20:00 was equivalent to 24 hour urine osmolality, while spot samples taken earlier in the day tended to overestimate 24 hour urine osmolality. This finding has practical utility for individuals, coaches, clinicians and researchers interested in evaluating 24 hour hydration without the burdens inherent in collecting 24 hour urine samples.