II. Epidemiology

  1. Represents 10% of Kidney Stones
  2. Occurs in 10-25% of gout patients
    1. Even higher risk with increasing Uric Acid levels (e.g. 50% Prevalence in those with Uric Acid >13 mg/dl)

III. Evaluation

IV. Types

  1. Pure Uric Acid Calculi
  2. Mixed Calcium and Uric Acid Calculi

V. Causes

  1. Primary cause
    1. Acidic urine (pH <5.5)
  2. Other causes
    1. Gouty Arthritis (confers 2 fold risk of calculi)
    2. Excessive dietary Purine intake (meats)
    3. End Ileostomy
    4. Insulin Resistance
      1. Results in impaired ammonia and citrate excretion
      2. Results in lower pH and increased urinary ammonia
      3. Increased Uric Acid crystallization

VI. Labs

  1. AM spot urine for Urine pH and Crystaluria
    1. Uric Acid stones form in acidic urine

VII. Imaging

  1. See Nephrolithiasis Imaging
  2. Limited Ultrasound for Acute Renal Colic
  3. Non-contrast Abdominal CT
  4. XRay Abdomen
    1. Pure Uric Acid Calculi are radiolucent
    2. May be visualized if mixed Calcium and Uric Acid

VIII. Management: Prevention of Uric Acid stone recurrence

  1. General
    1. Restrict dietary intake of Sodium and Protein
    2. Maintain Urine Output: over 2.5 liters per day
  2. Alkalinize urine (especially if Urine pH is low, acidic)
    1. Maintain Urine pH >5.5 (6.5 - 7.0 preferred)
    2. Potassium Citrate 10-20 mEq orally three times daily with meals (preferred) OR
    3. Calcium Citrate 500 mg, two tablets daily with meals OR
    4. Sodium Bicarbonate 650 mg tablets (7.74 meq each) two tablets orally three times daily
  3. Hyperuricosuria (>800 mg/day)
    1. Potassium citrate as above
    2. Allopurinol 100-300 mg/day
    3. Purine (Protein) restriction
    4. Reduce Urine Uric Acid excretion

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