Twenty-four-hour urine osmolality as a representative index of adequate hydration and a predictor of recurrence in patients with urolithiasis

To determine the value of 24-h urine osmolality (UOsm) as a representative index of adequate hydration and predictor of stone recurrence in patients with urolithiasis.

Medical records of consecutive patients presenting with renal or ureteric stones between 1994 and 2017 were retrospectively reviewed. Patients were grouped according to the results of 24-h UOsm (low ≤ 564 mOsm/kg H₂O, high > 564 mOsm/kg H₂O). Metabolic parameters and risk of stone recurrence were compared between the two groups.

The low urine concentration group were more likely to be older, to be female, and to have a lower body mass index and higher glomerular filtration rate than the high concentration group (each P < 0.005). A positive correlation was seen between 24-h UOsm and urinary calcium, sodium, uric acid, and magnesium excretion and 24-h specific gravity; a negative correlation was seen with 24-h urine volume. Stone-forming constituents, such as calcium and uric acid, were significantly higher in the high urine concentration group. Kaplan–Meier estimates showed that the low urine concentration group had a significantly longer stone recurrence-free period than the high urine concentration group (log-rank test, P < 0.001). In multivariate Cox regression analyses, 24-h UOsm was seen to be an independent risk factor for stone recurrence.

UOsm is a promising approach to assessing hydration and predicting stone recurrence in patients with urolithiasis. Maintaining UOsm < 564 mOsm/kg H₂O may reduce the risk of stone recurrence.