Diagnosing the Pathophysiologic Mechanisms of Nocturnal Polyuria

BackgroundDiagnosis of nocturnal polyuria (NP) is based on a bladder diary. Addition of a renal function profile (RFP) for analysis of concentrating and solute-conserving capacity allows differentiation of NP pathophysiology and could facilitate individualized treatment.ObjectiveTo map circadian rhythms of water and solute diuresis by comparing participants with and without NP.Design, setting, and participantsThis prospective observational study was carried out in Ghent University Hospital between 2011 and 2013. Participants with and without NP completed a 72-h bladder dairy. RFP, free water clearance (FWC), and creatinine, solute, sodium, and urea clearance were measured for all participants.ResultsThe study participants were divided into those with (n = 77) and those without (n = 35) NP. The mean age was 57 yr (SD 16 yr) and 41% of the participants were female. Compared to participants without NP, the NP group exhibited a higher diuresis rate throughout the night (p = 0.015); higher FWC (p = 0.013) and lower osmolality (p = 0.030) at the start of the night; and persistently higher sodium clearance during the night (p < 0.001). The pathophysiologic mechanism of NP was identified as water diuresis alone in 22%, sodium diuresis alone in 19%, and a combination of water and sodium diuresis in 47% of the NP group.ConclusionRFP measurement in first-line NP screening to discriminate between water and solute diuresis as pathophysiologic mechanisms complements the bladder diary and could facilitate optimal individualized treatment of patients with NP.Patient summaryWe evaluated eight urine samples collected over 24 h to detect the underlying problem in NP. We found that NP can be attributed to water or sodium diuresis or a combination of both. This urinalysis can be used to adapt treatment according to the underlying mechanism in patients with bothersome consequences of NP, such as nocturia and urinary incontinence.