Expansion
Supersaturation, low urine volume, and lack of inhibitors
Expansion
Three requirements:
- Supersaturation of the urine with the stone-forming salt
- Low urine volume, which raises concentration
- Deficiency of inhibitors, principally citrate and magnesium
Urine pH determines which stone forms:
| Stone | Favoured pH | Notes |
|---|---|---|
| Calcium oxalate | Any | Commonest, about 70 per cent |
| Calcium phosphate | Alkaline | Distal renal tubular acidosis |
| Uric acid | Acid (under 5.5) | Radiolucent; gout, dehydration |
| Struvite | Alkaline | Urease-producing organisms such as Proteus; staghorn calculi |
| Cystine | Acid | Cystinuria; a tubular transport defect |
Prevention follows the physiology: high fluid intake, thiazides to reduce calcium excretion, potassium citrate to raise citrate and alkalinise urine for uric acid and cystine stones, allopurinol for uric acid, dietary oxalate restriction, and maintaining normal dietary calcium, since low calcium intake paradoxically increases oxalate absorption and stone risk.