Uric acid is a weak organic acid – pKa 5.35 – 5.5 depending on source.
pKa predicts the pH that an acid will donate or accept a proton.
= at pH 5.35, half of uric acid will be undissociated uric acid, half will be urate anion.
Aiming for a pH of 7.0 to 7.2 for optimal dissolution (beware calcium phosphate stone formation if too alkaline urine) – patient should self monitor with dipsticks.
Classically quoted at 1 cm / month.
Use:
- Sodium bicarbonate 2 x 840 mg tabs, TDS, between meals
- Potassium citrate, 20 mEq TDS
- Ural sachets, 3-4 x day
Considerations:
- Contra-indicated in an obstructed kidney (partially obstructing may be OK, some reports suggest tamsulosin can aid in dissolution of ureteric stones)
- Also contraindicated if patient unwilling or unable to compliant with treatment and monitoring, or in opaque stones
- Adequate urine flow is necessary – aiming for > 2.5 L urine output daily
- Poor glycaemic control in diabetics is associated with poorer dissolution outcomes
- Therapies have high burden (logistically and side effects) and compliance may be low
- Ideally patient should monitor urinary pH
- Reduce dietary purines and proteins
Can add allopurinol especially in patients with hyperuricaemia – 100 – 300 mg /day.