Our prior investigations indicated that nocturnal polyuria (NP) may be driven not by an absolute increase in total sodium excretion, but rather by a temporal redistribution of sodium excretion toward the nighttime period. Conventional assessment of such circadian variation relies on 24-hour urine collection, which is cumbersome and not well suited for routine clinical practice. Although the Tanaka equation enables estimation of daily sodium excretion from spot urine samples, it does not capture diurnal fluctuations. To address this limitation, we devised a sodium excretion shift index (SESI) based on spot urine samples collected in the early morning (spot urine collected in the early morning; SEM) and in the evening (spot urine collected in the evening; SEE). The present study aimed to evaluate the relationship between SESI and NP, and to explore its potential as a practical clinical tool.

A total of 52 patients with nocturia (mean age 76.7 years; 41 men and 11 women) were enrolled in this study. The nocturnal polyuria index (NPi) and voided volumes were derived from 3-day frequency–volume charts. Sodium excretion in early morning and evening spot urine samples (SEM and SEE, respectively) was estimated using the Tanaka equation, and SESI was calculated as the ratio of SEM to SEE. Associations between SESI, NPi, and urinary parameters were assessed, and multivariable regression analysis was conducted to identify independent predictors.

The mean nocturnal voiding frequency was 2.9 episodes per night. Mean total, daytime, and nocturnal urine volumes were 1739 mL, 1002 mL, and 706 mL, respectively. The mean NPi was 0.4, while mean SEM and SEE were 194 mEq and 146 mEq, respectively, yielding a mean SESI of 1.4. SEM was significantly associated with nocturnal urine volume and NPi (both p<0.001), whereas SEE was significantly associated with daytime urine volume (p<0.001). SESI demonstrated strong associations with both nocturnal urine volume and NPi (p=0.002 and p<0.001, respectively), and remained an independent predictor in multivariable analysis (p<0.001). In contrast, estimated total sodium excretion was not significantly associated with NPi.

The present findings support the concept that NP is more closely linked to a shift in sodium excretion toward the nighttime rather than to total daily sodium output. SESI, derived from two spot urine samples using the Tanaka equation, appears to capture this altered temporal distribution of sodium excretion. Its strong association with nocturnal urine volume and NPi, including its independent predictive value, underscores its potential utility as a clinically relevant indicator of sodium excretion dynamics.

SESI is a simple, non-invasive, and clinically applicable index that may help identify patients with NP characterized by altered sodium excretion timing. This approach has the potential to inform individualized therapeutic strategies, including optimization of salt intake and diuretic timing, and may contribute to a more mechanism-based management of NP.

Continence 19S (2026) 102742
DOI: 10.1016/j.cont.2026.102742