# Estimated Osmolality by Measured Conductivity in 24 h Urine Renal Stone Patients: A Useful Tool for Monitoring Dietary Sodium and Protein Excess

## TL;DR

This study shows that measuring urine conductivity can help track dietary sodium and protein intake in kidney stone patients, offering a simple tool for monitoring.

## Contribution

The study introduces urine conductivity as a practical and reliable alternative to osmolality for monitoring dietary risks in kidney stone patients.

## Key findings

- Urine conductivity strongly correlates with osmolality and diuresis in kidney stone patients.
- A threshold of eUosm < 900 mOsm/day is linked to excessive sodium and protein intake.
- Conductivity measurements can be used to estimate osmolality with high accuracy.

## Abstract

Background/Objectives: Monitoring of 24 h urine analysis is currently used to assess diet-related stone risk factors due in most cases to low hydration and high osmole intake accounting for urine supersaturation. The aim of our study is to test whether urine conductivity could be a relevant surrogate marker of urine osmolality and a useful tool for monitoring salt and protein diets in primary care centers. Methods: 113 patients with kidney stone history referred for a routine evaluation of fasting and 24 h urine samples were included. Biochemical analysis of urine was performed, including measured osmolality (mUosm) and conductivity. Results: Among our population, 45% of patients have a low diuresis (high-risk group of stone recurrence) below the target of 2 L/day, with lower daily mUOsm and conductivity outflow compared to the low-risk patient group > 2 L/day (718 versus 852 mosm/Day, p < 0.0001, and 13,730 versus 17,890 mS/cm/day, p < 0.0001, respectively). Conversely to urine sodium and urea concentration, daily sodium and protein intake estimated by natriuresis and urea excretion are significantly lower in the high-risk group (p = 0.01 and <0.0001, respectively). In 24 h urine samples, osmolality and conductivity were strongly associated with diuresis. Moreover, a strong association between urinary osmolality and urine conductivity enables an estimated osmolality (eUosm) according to the following equation: eUosm = −41.656 + 0.057 × conductivity (r2 = 0.93; p < 0.001) with a 95% limit of agreement (LoA) ranging from −7.2% to +7.3%. An eUosm threshold value < 900 mOsm/day is independently associated with sodium and protein intake targets (odd ratio: 19.2 and 6.4-fold, respectively, p < 0.0001 and 0.01). Conclusions: 24 h urine measured conductivity appears to be a reliable, easy-to-use tool for the screening and monitoring of diet-related stone patients in primary care centers.