Composition of Urine Collected from Non-Stone-Forming Chinese Persons during Different Short-Term Periods of the Day

The purpose of this study in a small group of non-stone-forming Chinese persons was to measure the levels of supersaturation with calcium oxalate and calcium phosphate and pH with the aim of confirming if any of the different short-term urine samples were better for risk evaluation than a 24-h sample. Nine normal men and 1 woman collected urine during 4 periods of the day. Period 1 between 08 and 12 h, Period 2 between 12 and 18 h, Period 3 between 18 and 22 h, and Period 4 between 22 and 08 h. Each sample was analysed for calcium, oxalate, citrate, magnesium and phosphate, and estimates of supersaturation with calcium oxalate (CaOx) and calcium phosphate (CaP) were expressed in terms of AP(CaOx) and AP(CaP) index. An estimate of the solute load of CaOx was also calculated. Urine composition for 24-h urine (Period 24) was obtained mathematically from the analysed variables. Urine composition corresponding to 14-h urine portions 22–12 h (Period 14N) and 08–22 h (Period 14 D) were calculated. The lowest pH levels were recorded in Period 1 urine. The highest level of AP(CaOx) index was recorded during Period 1, and the product AP(CaOx) index × 107 × hydrogen ion concentration was significantly higher in Period 1 urine than in 24-h urine (p = 0.02). Also, the product SL(CaOx) × 107 × hydrogen ion concentration was significantly higher in Period 1 urine (p = 0.02). Low AP (CaP) index levels were recorded in Period 4, but also in all periods following dietary loads of calcium and phosphate. With the important reservation that the analytical results were obtained from non-stone-forming persons, the conclusion is that analysis of urine samples collected between 08 and 12 h might be an alternative to 24-h urine. The risk evaluation might advantageously be expressed either in terms of the product AP(CaOx) index × 107 × hydrogen ion concentration or the product SL(CaOx) × 107 × hydrogen ion concentration.