Purpose: We evaluated the intra-individual variability and reproducibility of nighttime urine production on wet nights and functional bladder capacity estimated by long-term home recordings of monosymptomatic nocturnal enuresis. In particular, the intention was to evaluate the validity of 1 versus 2 weeks of recording when estimating urine volume on wet nights and 1 versus 2 weekends of recording when estimating functional bladder capacity.
Materials and methods: We analyzed 120, 2-week home recordings of nighttime urine volume from patients with monosymptomatic nocturnal enuresis 6 to 16 years old (mean age 9.1) with at least 3 wet nights per week. Most patients were nonresponders or partial responders to desmopressin. Nighttime urine volume was estimated by weighing diapers before and after sleep, and measuring morning urine volume. Of the home recordings 62 included frequency volume charts for 2 weekends, which were evaluated for functional bladder capacity defined as the largest voided volume observed.
Results: No significant overall week-to-week differences were observed in average urine volume on wet nights and functional bladder capacity. There was a large intra-individual variability in all measured variables, which was most pronounced for functional bladder capacity and least pronounced for urine volume on wet nights. With regard to repeatability, the limits of agreement of urine volume on wet nights were -32% and 36% (95% confidence interval) as opposed to -54% and 48% for functional bladder capacity.
Conclusions: In this study intra-individual week-to-week estimates of average urine volume on wet nights demonstrated acceptable variability and repeatability in contrast to functional bladder capacity. A reliable estimate of urine volume on wet nights could be obtained by 7 nights of home recording, whereas 4 days of daytime recording were necessary when estimating functional bladder capacity. Similar studies of patients who respond to desmopressin are needed.