Diagnosing Nocturnal Polyuria from a Single Nocturnal Urine Sample

Background: Patients with nocturnal polyuria (NP) experience a unique surge in nocturnal diuresis rate during the early hours of sleep. Objective: To determine the diagnostic utility of the volume and osmolality of a single early nocturnal urine sample in detecting NP. Design, setting, and participants: Analysis of 1 AM urine samples obtained from two prospective observational studies at Ghent University Hospital involving participants recruited from a urology ambulatory care unit and those who consulted a continence clinic. Nocturic participants (one or more nocturnal void[s]; n = 176) were stratified based on the presence (n = 87) or absence (n = 89) of NP (>90 ml/h). Outcome measurements and statistical analysis: Receiver operating characteristic curves with Youden's Index were used to determine cutoff values for urine volume and urine osmolality (Uosm). Results and limitations: Individuals with NP demonstrated higher 1 AM volume (400 [interquartile range 300–515] vs 210 [160–300] ml, area under the curve [AUC] = 0.843, p < 0.001, cutoff = 350 ml) and lower Uosm (274 [201–348] vs 430 [320–664] mOsm/kg H₂O, AUC = 0.774, p < 0.001, cutoff = 314 mOsm/kg H₂O) than those without NP. In combining cutoffs, the criteria of either 1 AM volume ≥350 ml or Uosm ≤314 mOsm/kg H₂O were 85% sensitive and 75% specific for NP, while criteria of both 1 AM volume ≥350 ml and Uosm ≤314 mOsm/kg H₂O were 60% sensitive and 92% specific for NP. Comparable AUC values, sensitivities, and specificities were observed in both men and women. Limitations include post hoc design and a relatively small study sample. Conclusions: Individuals with NP are more likely than those without NP to produce a higher volume of more dilute urine (ie, “aquaresis”) in the early hours of sleep. Analysis of easily measurable parameters of the first nocturnal void (for which 1 AM values serve as a surrogate) in men and women with nocturia can predict a diagnosis of NP with a reasonably high degree of sensitivity and specificity. Patient summary: Urologists often try to understand the specific reason why people wake up to urinate at night by asking them to record the amount of urine they make every time they go to the bathroom (also known as a “voiding diary”) during the nighttime as well as the daytime—typically for a total of 1–3 days. In this study, we showed that an analysis of the composition of the urine that people produce when they first wake up to urinate at night might be sufficient to determine whether their symptoms are caused by excessive urine production or something else, and some people might find this urine study easier than keeping a voiding diary. Individuals with nocturia owing to nocturnal polyuria produce a large volume of dilute urine in the early sleep period. Easily measurable parameters of the first nocturnal void can predict nocturnal polyuria with a reasonably high degree of sensitivity and specificity.