Spot Urine Oxalate Testing in a Controlled Trial of Forced Hydration and Spinach Consumption

To assess how spot urinary oxalate measurements may uncover post-prandial spikes in oxalate excretion and the impact of hydration on urinary oxalate concentration after a high-oxalate load (HOL).

Healthy non-stone formers (n = 21) were prospectively enrolled and collected 48 hours of consecutive urine on low oxalate diet. At noon on both days, participants consumed HOL (10 oz. spinach) and voided every 2 hours for an 8-hour post-HOL interval. On Day 2, participants increased water intake (12 oz/2 hours) for the 8 hours post-HOL. Portions of each void ("spots") were used to measure urinary oxalate, and hyperoxaluric void (HOV) was defined as oxalate excretion >1.67 mg/hour or >40 mg/day.

Mean urine oxalate excretion ranged from 17 to 97 mg/24 hours, correlating to 2%-12% absorption of ingested oxalate. Although peak urinary oxalate varied by individual, most (91%) had one or more HOV, reaching peak oxalate excretion ~7 hours following HOL. Day 1 urine oxalate concentration spikes decreased up to 5-fold (P = .014) by increasing water intake on study Day 2. Individuals with hyperoxaluria had distinctive spot excretory oxalate patterns that allowed for categorization by either absorptive or endogenous type.

Following a spinach load in non-stone formers, total and peak urinary oxalate excretion vary greatly. Forced hydration lowers urinary oxalate concentration regardless of the amount of oxalate excreted, confirming the importance of fluid intake. Hyperoxaluric controls had unique patterns of spot oxalate testing, suggesting a role for spot urinary oxalate monitoring in this population.