P176 SPOT URINE FORMULAS OVERESTIMATE DIETARY SODIUM INTAKE DURING LOW SODIUM CONSUMPTION

Background and objective: Long-term sodium balance studies have revealed a phenomenon of temporary storage of sodium in tissues. Consequently, relying on an individual's single 24-hour urine collections may lead to an unreliable estimation of sodium intake. Our recent research has shown that the accuracy of repeated spot urine collections is equal to that of a single 24-hour urine collection at a sodium intake level of 5 gram/day. Since no data is available about the performance at recommended sodium intake (i.e., <2 grams/day), we determined their accuracy during low sodium consumption. Methods: We performed a post-hoc analysis on data from a metabolic ward study in which 16 healthy male adults consumed a diet with a fixed sodium content of 50 or 200 mmol/day for 7 days. Each day, urine was collected in 4 intervals (7:00-13:00 h, 13:00-19:00 h, 19:00-23:00 h, 23:00-07:00 h). We estimated sodium intake by the European Toft formula using urines of the same interval on 3 consecutive days during steady state (day 5-7). In sensitivity analyses we applied the Tanaka and Kawasaki formulas. Results: During a 200-mmol sodium intake, the Toft formula estimates appeared the most accurate, deviating 10 [IQR 3-14], 8 [6-19], 36 [16-49] and 20 [10-43] mmol from the intake for intervals 7:00-13:00 h, 13:00-19:00 h, 19:00-23:00 h, 23:00-07:00 h, respectively (Figure 1A). This accuracy did not significantly differ from a single 24-hour urine (20 [12-55] mmol). However, at a 50-mmol sodium intake, the Toft formula estimates deviated 62 [IQR 45-69], 75 [67-97], 99 [90-109] and 51 [45-61] mmol from the intake for similar intervals (Figure 1B). These estimates were significantly less accurate than a single 24-hour urine (9 [6-23] mmol). Although the Tanaka equation outperformed the Toft equation, this formula was also significantly inferior to a 24-hour urine collection. Conclusions: Repeated spot urine collection substantially overestimates sodium intake when sodium intake is low, whereas it performs similarly to a 24-hour urine collection at an average sodium intake.