Quantitating nocturia: a study into the recording of solute and water excretion to determine causation

Nocturia is common but the clinical assessment of its severity and cause rarely involves any biochemical analysis. Investigating the cause of nocturia needs to be informed by the overall 24 h fluid and solute excretion patterns. The aim of this study was to establish a practical method of monitoring the renal excretion of water and solutes over a complete 24 h cycle.

The excretion patterns of sodium, volume and osmoles were assessed in 89 healthy control subjects over a 24 h period by sampling each voiding from the 24 h collection and then using the total urine creatinine as the denominator. A group of 21 patients under investigation for sleep-disordered breathing (SDB: a group of disorders known to increase the risk of nocturia) were also studied to determine comparative excretion patterns.

Reference excretion patterns of sodium, volume and osmoles were described. Patients under investigation for SDB had overall a significant (P < 0.001) increase in urine sodium excretion at night (nocturnal natriuresis) matched by an increased osmotic excretion and accompanied by a significantly increased nocturnal urine volume (P < 0.001).

Breaking down a 24 h urine collection into voided aliquots provides practical information on the pattern of water and solute excretion. Such patterns may assist in identifying the underlying mechanism of significant nocturia in individual patients presenting with this symptom, and could be used as a method of monitoring treatment.