Is it effective to do mathematical analysis for the etiology of nocturia using the nocturia indices derived from the frequency volume chart?

The aim of this study was to determine the problems involved in using a mathematical calculation to analyze the pathophysiology of nocturia using a frequency volume chart (FVC). In this retrospective study, we reviewed 253 patients who completed the FVC during ≥3 days for nocturia from January 2017 to February 2019. The etiology of nocturia was defined as a combination of four pathophysiologies, including nocturnal polyuria (NP), decreased bladder capacity (dBC), decreased nocturnal bladder capacity, and global polyuria. To analyze the differences according to diagnostic criteria for NP and dBC, two definitions were classified. Definition 1: NP is nocturnal polyuria index (NPi)>0.35 and dBC as maximal voided volume (MVV)<325 mL. Definition 2: NP is NPi=0.20-0.33 depending on age and dBC as MVV<200 mL. The MVV in the FVC for ≥3 days was significantly higher than that for 1 day (330 mL vs. 400 mL, p<0.001). During the entire FVC period, all pathophysiology remained unchanged in only 16.6% of cases (inter-day variation). Of 323 days in which nocturia occurred ≥2 per night, 118 days (36.5%) full bladder voiding of nocturia was not the same for each day (inter-nocturnal variation). According to definitions 1 and 2, the serial changes of NP and dBC showed different patterns. The mathematical calculation used in analyzing the pathophysiology of nocturia using an FVC has problems such as differences according to the duration of FVC and inability to express inter-day and inter-nocturnal variations. Therefore, this mathematical calculation is not helpful when determining the treatment modality for nocturia.