V6-06: Urine acidity is the most important factor in uric acid stone formation: an illustration

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INTRODUCTION

To illustrate the role of urinary pH and uric acid concentration in uric acid (UA) stone formation.

METHODS

We used five aliquots of fresh 24 hour urine collection from a non–stone forming patient whose initial urinary concentration of UA was 300 mg/L with an initial urine pH of 6.75._x000D_ We used a solution of potassium urate to assess the role of UA concentration, by adding increments of this solution to the different aliquots while maintaining the pH at 6.75. Then, in order to assess the impact of urinary pH, the pH in the aliquots with varying UA concentration was dropped to 5.0. When pH is below 5.5 (pKa of UA), urate is replaced by undissociated UA, which is very insoluble. Finally, to assess the role of alkalinization, the pH was raised back to its initial value (6.75) where the majority of UA is in the form of anionic urate which is very soluble._x000D_ We observed the effects of varying UA concentration at these different times:_x000D_ - Initial pH was kept constant at 6.75._x000D_ - pH was lowered to 5.0_x000D_ - pH was increased back to its initial value after being reduced to 5.0

RESULTS

After 3 hours incubation at 37°C at varying UA concentration, no precipitation was observed when the pH was maintained at its initial value (6.75). When the pH was lowered to 5.0, the formation of UA crystals was observed after 3 minutes, this occurring faster with higher initial UA concentration._x000D_ When the pH was increased back to its initial value (6.75) by the addition of potassium hydroxide, UA crystals dissolved, this occurring faster at lower UA concentration.

CONCLUSION

Urinary acidity is the main factor in the UA crystallization compared the hyperuricosuria. Urinary alkalization is the cornerstone for prevention and treatment of uric stones.

Funding: none