Urinary supersaturation on fractioned urine collections: which urine sample can explain better the variability observed on 24-h urine? A proof-of-concept study

Due to the difficulty of collecting 24-h urines in the stone-forming patient, some authors have suggested other types of urine collection, but their usefulness is not yet well studied. The objective of this study is to evaluate the variation of urinary supersaturation (SS) throughout the day and to analyze whether timed urine collections offer accurate information. 48 urine samples were collected from 12 young adults. Each 24-h urine was collected on 7 2-h urine fractions and a 10-h overnight sample. Solute concentrations and SS for calcium oxalate (CaOx), calcium phosphate (CaP), and uric acid (UA) were determined. Linear regression and relative importance of predictors were used to determine the percentage of R² attributed to each timed collection (individual SS). 43 24-h urine samples were included in the study. The highest SS values were: for CaOx, night period and first morning urine; for CaP, between 2 and 6 pm and at night; for UA, between 8 am and 12 pm. For CaOx, the SS from the samples between 8 pm and 8 am accounted for more than 40% of the R²; for CaP, the results were more equally distributed throughout the day, and for UA, the SS values from 12 to 4 pm accounted for more than 45% of the observed variability. In conclusion, urinary SS varies throughout the day, being higher for CaOx and CaP at night, and in the early morning for UA. For CaOx and UA, the overnight and 12–4 pm urine samples, respectively, contribute most to the variability observed in the SS of 24-h urine.