Assessment of Urinary Inhibitor or Promoter Activity in Uric Acid Nephrolithiasis

Purpose We assessed decreased inhibitor activity or increased promoter activity in the urine of idiopathic uric acid stone formers compared to nonstone formers independent of urinary pH. Materials and Methods A total of 30 idiopathic uric acid stone formers, and 9 obese and 12 lean nonstone formers collected 24-hour urine while on a metabolic diet. Three urine aliquots per subject were used to assess spontaneous nucleation (de novo crystal formation), crystal growth using a 0.1 mg/ml anhydrous uric acid seed and steady-state uric acid solubility (the maximum amount of uric acid dissolvable in urine) using a 5 mg/ml uric acid seed. All experiments were performed for 6 hours at a constant pH of 5.0. Uric acid concentration was measured in filtered aliquots at 0, 3 and 6 hours. Results At baseline 24-hour urinary pH was significantly lower and uric acid saturation was significantly higher in idiopathic uric acid stone formers. No significant spontaneous nucleation developed and similar uric acid steady-state solubility was reached in the 3 groups. Idiopathic uric acid stone formers and lean nonstone formers showed a similar decrease in uric acid concentration during crystal growth. Obese nonstone formers started with a higher uric acid concentration and consequently demonstrated a greater decrease in the uric acid concentration for crystal growth. Conclusions This study suggests that there is no significant difference between idiopathic uric acid stone formers and nonstone formers in promoter or inhibitor activity in whole urine against uric acid stone formation when urine pH is maintained constant. The findings suggest that uric acid stone formation is dictated by high urinary saturation with respect to uric acid, which is driven primarily by low urine pH.