According to the ICS, the maximum voided volume (MVV) is defined as the highest voided volume recorded during an assessment period, and usually equals bladder capacity. Herein we define two new terms; the voiding frequency index (VFI) and the functional bladder capacity index (FBCI). The VFI is the minimum number of times a person must void based on 24-hour voided volume (24HVV) and MVV. We define the number of “extra voids”, the difference between the actual number of voids (ANV) in 24 hours recorded in a bladder diary and the VFI, as the FBCI. The purpose of this study is to develop FBCI values for patients with overactive bladder (OAB), asymptomatic persons, and to determine how FBCI is influenced by total urine output.

This was a retrospective study of 262 men and 138 women diagnosed with OAB after having completed a 24-hour bladder diary and lower urinary tract symptom score questionnaire using a smartphone app* from 2015-2019. Patients with incomplete diaries were excluded. From the diary, we extracted the 24HVV, MVV, and ANV per 24-hours. VFI was calculated by dividing the 24HVV by the MVV to express the absolute minimum amount of voids required in 24-hours. FBCI was calculated by subtracting the VFI from the ANV. 

Mean values for ANV, VFI, and FBCI were calculated in our sample. To provide a comparison to normative data from asymptomatic men and women without LUTS, we calculated ANV, VFI, and FBCI from published reference voiding values; 24HVV, ANV, MVV by Tissot, Amundsen, Diokno, Webster, Parsons, Cardozo, and Coats [1,2]. Additionally, the cohort was divided into three distinct urinary phenotype groups; normal (1000-2500 mL per 24 hours), oliguria (<1000 mL per 24 hours), and polyuria (>2500 mL per 24 hours) based on their 24HVV. Mean values for ANV, VFI, and FBCI were compared across each of the three urinary phenotypes. 

*weShare® URO from Symptelligence.com

A total of 400 patients, ages 14-93, evaluated for OAB completed a 24-hour bladder diary and LUTSS questionnaire. Of these, 19 patients were excluded due to inaccurate bladder diary and LUTSS data. When comparing our sample of 381 patients to the reference values, the mean values for VFI, ANV, and FBCI in our sample were greater **. 
 
The 381 patients were classified into three distinct urinary phenotype groups based on their 24HVV; 230 were placed in the normal group, 47 were placed into oliguria, and 104 into polyuria. We compared the mean FBCI in each urinary phenotype via a single factor ANOVA test in the total sample, and then across men and women (figure 2). In the total sample, the polyuria group had the highest FBCI, followed by the normal and oliguria groups. In men and women, we found consistent results, but with less statistical significance. 

**Statistical significance could not be evaluated since Tissot et al. original sample could not be compared to our sample.

All people, whether experiencing LUTS or not, void more often than absolutely required over 24 hours based on MVV, but this matter has not been studied previously. Through analysis of Tissot et al data, we established a baseline of extra voids in patients without LUTS to be compared to those with OAB in our sample. Those without LUTS void an average of 3 times more than the minimum required by the VFI, and those with OAB void an average of over 4 times more. Additionally, when comparing FBCI across urinary phenotypes, we found significance statistically and clinically. Most importantly, those with OAB and polyuria void an average of over 5 times more than minimum.

FBCI is a quantitative metric for the “extra voids” in a 24-hour period. Empirically, the larger the FBCI, the greater the likelihood a patient voids because of reasons unrelated related to bladder capacity. We observed a general trend that higher urinary output, as in polyuria, leads to greater amounts of extra voids, and this pattern seems to be exaggerated in certain conditions such as OAB in this study. FBCI as a metric may be useful in studies to quantitate the effect of behavior, detrusor overactivity, and sensory disorders of the bladder on voiding patterns, and prompting greater attention and research into bladder-independent determinants of voiding.

Tissot W, Amundsen CL, Diokno AC, Webster GD, Coats AC; Bladder Diary Research Team. Bladder diary measurements in asymptomatic males: frequency, volume per void, and 24-hr volume. Neurourol Urodyn. 2008;27(3):198-204.
Amundsen CL, Parsons M, Tissot B, Cardozo L, Diokno A, Coats AC. Bladder diary measurements in asymptomatic females: functional bladder capacity, frequency, and 24-hr volume. Neurourol Urodyn. 2007;26(3):341-9.