Upper metastable limit osmolality of urine as a predictor of kidney stone formation in children

Abstract

High fluid intake has been universally recommended for kidney stone prophylaxis. We evaluated 24-h urine osmolality regarded as the best biomarker of optimal hydration and upper metastable limit osmolality after water evaporation from urine sample to the onset of spontaneous crystallization and its usefulness as a new risk index that would describe an individual lithogenic potential. We collected 24-h urine from 257 pediatric patients with kidney stones and 270 controls. After volume and osmolality assessment, the urine samples were subjected to volume reduction in vacuum rotavapor continued to the onset of an induced urinary crystallization. The upper metastable limit osmolality of urine sample was calculated based on its initial osmolality value and the amount of water reduction. Pediatric stone formers presented with higher urine volume and lower urine osmolality than healthy controls. Despite that, their urine samples required much lower volume reduction to induce the spontaneous crystallization than those of controls. The ROC analysis revealed an AUC for the upper metastable limit osmolality of 0.9300 (95% CI 0.9104-0.9496) for distinguishing between stone formers and healthy subjects. At the cutoff of 2696 mOsm/kg, the test provided sensitivity and specificity of 0.8638 and 0.8189, respectively. 24-h urine osmolality provided the information about current hydration status, whereas evaporation test estimated the urinary potential to crystalize dependent on urine composition. Upper metastable limit osmolality may estimate the individual lithogenic capability and identify people at risk to stone formation when exposed to dehydration.

Response to the letter to editor: 24-h urine osmolality should be used in combination with other urine parameters in urolithiasis patients

“…The most important issue raised by the author is that 24-h UOsm, as an index of hydration status, could not provide sufficient information in regard to urine PH, solute saturation and individual lithogenic capability. We also agree with the author's suggestion that 24-h UOsm should be used in combination with the other urine parameters such as pH, solute saturations, and upper metastable limit osmolality given the complexity of stone disease [2].…”

“…We also concede that our UOsm cut-off value may not be possible to generalize to other populations. As mentioned by the authors, upper metastable limit osmolality might evaluate individual lithogenic capability and identify patients needing more aggressive hydration therapy [2].…”

LITHORISK.COM: the novel version of a software for calculating and visualizing the risk of renal stone

“…This explains why several urine-based indices have been proposed trying to simplify the assessment of the stone risk. These include the Tiselius Index [ 28 ], the Robertson PSF index [ 29 ], the Bonn-Risk Index [ 30 ], the upper limit of osmolality proposed by Porowski et al [ 31 ] and maybe many others. However, none of these has widely diffused, so that they warrant confirmation by clinical trials.…”

Specific populations of urinary extracellular vesicles and proteins differentiate type 1 primary hyperoxaluria patients without and with nephrocalcinosis or kidney stones

“…Urine osmolality tended to be lower in PH1 patients with NC or KS compared to those without NC or KS. Previous studies have also reported decreased urine osmolality in patients with NC and/or KS [20,21]. Since a lower urine concentration should protect against rather than favor crystallization, these changes are likely a result rather than cause of NC.…”