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Kavouras S.A. et al. 2021

Urine osmolality predicts calcium-oxalate crystallization risk in patients with recurrent urolithiasis

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Abstract



Background/Objectives:

The aim was to investigate the validity of osmolality from 24-h urine collection in examining the risk for calcium-oxalate (CaOx) kidney stone formation in patients with recurrent urolithiasis.

Subjects/Methods:

Three hundred and twelve subjects (males/females: 184/128) from France with a history of recurrent kidney stones from confrmed or putative CaOx origin were retrospectively included in the study (46±14 years, BMI: 25.3±5.0 kg·m−2). Tiselius’ crystallization risk index (CRI) was calculated based on urinary calcium, oxalate, citrate, magnesium, and volume from 24-h samples. The diagnostic ability of 24-h urine osmolality to classify patients as high risk for kidney stone crystallization was examined through the receivers operating characteristics analysis. High risk for CaOx crystallization was defned as CRI>1.61 and>1.18, for males and females, respectively. The accuracy of urine osmolality to diagnose risk of CaOx stone formation (AUC, area under the curve) for females was 84.6%, with cut-of point of 501 mmol·kg−1 (sensitivity: 83.3%, specifcity: 76.0%). Males had AUC of 85.8% with threshold of 577 mmo·kg−1 (sensitivity: 85.5%, specifcity: 77.6%).

Results:

A negative association was found between 24-h urine volume and osmolality (r=− 0.63, P<0.001). Also, a positive association was found between 24-h urine osmolality and CRI (r=0.65, P<0.001), as well as urea excretion with CRI (r=0.37, P<0.001). In conclusion, urine osmolality>501 and>577 mmol·kg−1, in female and in male, respectively, was associated with a risk for CaOx kidney stone formation in patients with a history of recurrent urolithiasis.

Conclusion:

Thus, when CaOx origin is confirmed or suspected, 24-h urine osmolality provides a simple way to define individualized target of urine dilution to prevent urine crystallization and stone formation.


A word from our expert, Dr Mariacristina Vecchio :

"For the first time, we demonstrated that in patients having CaOx stones, 24-h urine osmolality is a simple way to define individualized target of urine dilution to prevent urine crystallization and stone formation. Specifically, according to our analysis, urine osmolality > 501 and > 577 mmol·kg−1, in female and in male, respectively, was associated with a risk for CaOx kidney stone formation in patients with a history of recurrent urolithiasis. Thus, these values of urine osmolality, could be used as a goal for providing individualized fluid intake recommendation to prevent stone recurrences."