Nocturnal polyuria: excess of nocturnal urine production, excess of definitions: influence on renal function profile

Purpose: This study aimed to identify important differences in renal function profile, and potential water and sodium diuresis cutoffs among participants with nocturnal polyuria according to nocturnal polyuria definitions. Materials and Methods: This post hoc analysis was based on a prospective study in which participants completed a bladder diary, collected urine and provided a blood sample. With an age dependent nocturnal polyuria index greater than 20% to 33% as the referent 4 definitions of nocturnal polyuria were compared, including 1) nocturnal polyuria index greater than 33%, 2) nocturnal urine production greater than 90 ml per hour and 3) greater than 10 ml/kg, and 4) nocturia index greater than 1.5. Results: In 112 male and female participants significant differences in baseline characteristics and bladder diary parameters were found according to definition. Diuresis rate, free water clearance and sodium clearance had similar 24-hour courses in the subgroups with and without polyuria by each definition. The range varied more in the subgroup with vs without polyuria, especially at night for diuresis rate and free water clearance. At night the latter decreased in the polyuria subgroup based on each definition (p < 0.001 to 0.045). A significant difference vs the no polyuria subgroups was found only for urine production greater than 90 ml per hour and polyuria index greater than 20% to 33%. For each definition sodium clearance remained high in the polyuria subgroup, which differed significantly from the no polyuria subgroups (p < 0.001 to 0.030). Free water and sodium clearance cutoffs ranged from -0.65 to -0.85 ml per minute between 12 and 2 a.m., and 0.65 to 0.77 ml per minute between 3 and 5 a.m., respectively, with large sensitivity and specificity differences according to definition. Conclusions: There were important differences when comparing participants with vs without nocturnal polyuria by definition. The renal function profile indicating the pathophysiological mechanism of nocturnal polyuria did not seem to be influenced by definition but free water clearance and sodium clearance cutoff sensitivity differed substantially. These results must be confirmed in a larger homogeneous sample