Background: Because 24-hour urine collections are cumbersome, many studies have evaluated the use of spot urine samples as a substitute, mostly finding poor concordance between the two. Daily variation in stone parameters probably contributes to the lack of concordance, but specific variation in various stone parameters is not well delineated. The variations likely lead to peaks and troughs, which can increase the risk of stone formation. Methods: We prospectively recruited 20 nonstone-forming patients, recording their total fluid intake over 24 hours and collecting voids at first morning, 9 to 10 A.M., 1 to 2 P.M., and 4 to 5 P.M. for evaluation of pH, specific gravity, calcium, citrate, and creatinine. Participants were then asked to double their fluid intake and take a daily True Lemon supplement over the course of the next 3 days. Urine was recollected postintervention. Results: Baseline [citrate]/[creatinine] increased throughout the day such that the 5 P.M. level was significantly higher compared with first void (0.58 vs 0.42, p = 0.027); [calcium]/[creatinine] daily variation was not statistically significant, but showed a distinct pattern that was present in both sets of collections. Daily [calcium]/[citrate] variation was significantly (p = 0.004) and consistently highest in the early morning on both day 1 (0.43) and day 4 (0.45). There was no significant variation in specific gravity and pH. Increasing fluid intake and citrate supplementation increase the daily variation in pH and [citrate]/[creatinine], but did not increase the values compared with their respective preintervention void times. There was also no detectable postintervention effect on [Ca]/[creatinine] or specific gravity. Conclusions: Urinary citrate concentration follows a circadian pattern, while urinary calcium has a diurnal excretion pattern. [Calcium]:[citrate] is highest in the early morning, indicating a high-risk time of day for stone formation. Spot urine samples identify a key time of day, which 24-hour urine collections may miss, for clinical monitoring.
Keywords: calcium; citrate; spot; urine; variation.