24-hour urinary oxalate testing is a key component of comprehensive metabolic evaluation but cannot measure transient oxalate spikes. These spikes may confer an increased risk of stone formation. We sought to assess (1) urine oxalate excretion following high dietary oxalate load (HOL), and (2) the impact of increasing fluid intake on oxalate concentrations after HOL.

Methods:

With IRB approval, seven healthy non-stone forming subjects were prospectively enrolled to maintain a low oxalate diet and collect voids over 48 hours On Day 1, subjects were asked to consume a HOL (10 oz spinach, 820 mg oxalate) and to collect voids every 2 hr for the 8 hr post-HOL interval. On Day 2, subjects increased water intake (12 oz/2 hr) for the 8 hr post-HOL interval. Oxalate levels were determined by assay. Hyperoxaluric voids (HOVs) were those with estimated excretion rate 1.67 mg/hr.

Results:

Following HOL on Day 1, mean peak oxalate excretion rate was 3.10 ± 2.43 mg/hr The 8 hr post-HOL interval on Day 1 accounted for 46.7% of the respective 24 hr oxalate excretion. On Day 1, 71.4% (n=5) had ≥ 1 HOV following HOL, with 80% of HOV being within the 8 hr post-HOL interval. With increased post-HOL water consumption on Day 2, there was a trend towards attenuation of change in oxalate concentration post-HOL.

Conclusions:

Significant spikes in urinary oxalate excretion occur following HOL, which may be attenuated by increasing water intake. These findings suggest the need for temporal monitoring of urinary oxalate levels.