Night-to-Day Ratio of Excreted Urinary Sodium Concentration Using Spot Urine in Older Adults With Nocturnal Polyuria.

Background: The diurnal rhythm in water and solute diuresis is known to be disturbed in older adults with nocturia due to nocturnal polyuria. We estimated the prevalence of increased natriuresis in nocturia due to nocturnal polyuria by comparing individuals with and without nocturnal polyuria.

Methods: We calculated the night-to-day ratio of excreted urinary sodium and potassium concentrations adjusted for urinary creatinine concentration and urinary osmolality using spot urine samples collected at 2 PM (daytime) and 6 AM (nighttime) and compared results among controls with nocturia and paitents with nocturia with and without nocturnal polyuria.

Results: Among 83 patients aged 50 to 86 years, the mean night-to-day ratio of excreted urinary sodium and potassium concentrations in the nocturia group with nocturnal polyuria (n = 40) was 2.5, which was significantly higher (p = 0.034) than those in the control group (n = 23; 0.7) and the nocturia group without nocturnal polyuria (n = 20; 0.9). After adjustment for age, no difference in potassium and urinary osmolality existed among groups. Thirteen patients (33%) in the nocturia group with nocturnal polyuria showed increased natriuresis when a 2.0 night-to-day ratio of excreted urinary sodium was applied as a cut-off for increased natriuresis, whereas only 3 (7%) patients in the combined groups without nocturnal polyuria had increased natriuresis (p = 0.005).

Conclusions: One-third of patients with nocturia due to nocturnal polyuria had increased natriuresis. Assessment of a night-to-day ratio of excreted urinary sodium using spot urine samples could facilitate a better understanding of the underlying pathophysiology and optimal management of nocturnal polyuria.