First voided volume: A novel approach to characterize nocturia

Nocturnal polyuria syndrome (NPS) denotes nocturnal polyuria (NP) in the absence of identifiable contributory factors. The trajectory of nocturnal urine production (NUP; typically expressed as ml/hour) may be useful in delineating between NP patients with versus without NPS, but changes in absolute urine volume, the directly measured substrate for behavioral and pharmacologic interventions targeting nocturnal urine production, have not been well characterized. This study compares the ratio of the first nocturnal voided volume (FNVV) to the nocturnal average voided volume (NAVV) in patients with versus without NPS.

Secondary analysis of 24-h voiding diaries from male patients greater than or equal to 18 years of age with two or more nocturnal voids and NP using two different criteria for NP: NUP greater than or equal to 90 ml/h and nocturnal polyuria index (NPi) greater than or equal to 0.33. Patients with diabetes insipidus and CPAP-adherent obstructive sleep apnea (OSA) were excluded. Patients were divided into 2 groups: secondary NP (OSA, congestive heart failure, and chronic kidney disease) and NPS (absence of edema, diuretic use, and the aforementioned comorbidities). FNVV was defined as the volume of urine accompanying the first nocturic episode. NAVV was defined as nocturnal urine volume/(number of nocturnal voids + 1). The nocturnal urine trajectory ratio (NUTR) was defined as FNVV/NAVV.

At NUP greater than or equal to 90 ml/h, NUTR was significantly greater in patients with (n = 73) versus without (n = 28) NPS (1.10 [0.89-1.33] vs. 0.91 [0.55-1.15], p = .012). At NPi greater than or equal to 0.33, NUTR was likewise significantly greater in patients with (n = 92) versus without (n = 32) NPS (1.09 [0.90-1.33] vs. 0.91 [0.57-1.17], p = .010).

The volume of urine produced in the early hours of sleep is central to identification of NPS in patients with nocturia.