The pathogenesis of nocturia centers on a fundamental mismatch between nocturnal urine production and small bladder storage capacity (SBC). A 24-hour voiding diary is the gold standard for differentiating these mechanisms, but requires careful documentation of the timing and volume of every void across a 24-72-hour period, such that patient adherence remains a pervasive obstacle in the real-world clinical setting. Assessment of voiding diary data from the hours of sleep can, in of itself, support a diagnosis of nocturnal polyuria (NP) on the basis of nocturnal urine production rate [1]. Conversely, a diagnosis of SBC from the voiding diary is typically based on the single largest volume across the diary record, referred to as the 24-hour ‘maximum voided volume’ (MVV). While the nocturnal MVV (NMVV) can, by definition, be used to exclude SBC, the utility of this parameter to rule in a diagnosis of SBC remains unclear. Accordingly, this study aims to determine the specificity of NMVV for SBC among men with nocturia.

Analysis of voiding diaries from men aged ≥18 years who were treated for lower urinary tract symptoms at an outpatient urology clinic from 2008-2019. Local Institutional Review Board approval with a waiver of informed consent was granted for retrospective analysis of routinely collected institutional data. Complete voiding diaries showing clinically-significant nocturia (≥2 nocturnal voids) were included for analysis. Included diaries were stratified based on the presence/absence of SBC (defined as a 24-hour MVV ≤200 ml) [2] and positive/negative nocturnal test for SBC (defined as NMVV ≤200 ml). Subgroup analyses were subsequently performed in patients with and without comorbid NP (defined as a nocturnal urine production rate >90 ml/hour) [1,3]. Specificities of the nocturnal test for SBC were calculated for each grouping.

SBC was present in 98 (27.5%) of the 356 voiding diaries which met the criteria for inclusion. Among voiding diaries without SBC, the nocturnal test was truly negative (i.e., NMVV >200 ml) in 225 cases and falsely positive (i.e., NMVV ≤200) in 33 cases, corresponding to 87.2% specificity. In the presence of NP (n=179), the nocturnal test for SBC was truly negative in 158 cases and falsely positive in 8 cases, with a specificity of 95.2%. In the absence of NP (n=177), the nocturnal test for SBC was truly negative in 67 cases and falsely positive in 25 cases, corresponding to 72.8% specificity.

By definition, NMVV >200 ml mathematically rules out a voiding diary-based diagnosis of SBC, but SBC cannot necessarily be excluded when NMVV is ≤200 ml. The major finding of the present study is that NMVV ≤200 ml is indeed rather specific for 24-hour SBC, particularly in the setting of concomitant NP, such that a positive nocturnal test for SBC (i.e., a nocturnal-only voiding diary showing all nocturnal voids at or below 200 ml) may reliably rule a diagnosis of SBC based on the 24-hour voiding diary. Further research is needed to corroborate these findings in view of post-void residual volumes which, in summation with voided volumes, more accurately reflect functional bladder capacity.

Nocturnal voided volumes provide valuable insight into voided volumes across the 24-hour period. By definition, a NMVV >200 ml excludes SBC, but a NMVV ≤200 ml may also reliably rule-in a diagnosis of SBC, particularly in patients with comorbid NP. These findings support the use of nocturnal-only voiding diaries in the initial evaluation of nocturia owing to NP.

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