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AIM This study determined the specific reference values for urinary phosphorus excretion in healthy children and adolescents aged 2-18 years and evaluated whether they changed with age during growth and were gender dependent. METHODS We enrolled 3913 healthy children and adolescents aged 2-18 years to this study. The study population was divided into age groups, and the analysis was performed in one-year periods, separately for boys and girls. Urinary phosphorus excretion was analysed using four categories: P1 in mmol/24 hour units, P2 in mmol/kg/24 hours, P3 in mmol/1.73 m2 /24 hours and P4 in mmol/mmol creatinine. RESULTS Clear differences in urinary exertion for girls and boys were observed as well as systematic changes with age. The boys presented with significantly higher daily urinary phosphorus excretion independent of its manner of expression (p < 0.001). The median urinary phosphorus (P1) rose with age (p < 0.001). Percentile tables of phosphorous exertion are presented. CONCLUSION This was the largest study of urinary phosphate excretion based on a randomly selected sample of girls and boys aged 2-18 years. It highlights the importance of determining phosphorus reference values for children of different ages to provide early diagnosis and treatment for urolithiasis.

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.