Kidney Stone Primer

When a patient needs a metabolic evaluation for kidney stones the twnety-four hour urine should include at minimum:

  • Calcium
  • Oxalate
  • Citrate
  • Uric Acid
  • Volume
  • pH
  • Creatinine

A complete evaluation adds:

  • Sodium
  • Potassium
  • Chloride
  • Urea nitrogen
  • Phosphorous
  • Magnesium
  • Ammonia
  • Sulfate

Hypercalciuria is defined as over 300 mg/day in a man and over 250 mg in a woman. Normal urinary calcium is 150-170 mg per day.

Urine oxalate over 90 mg/day should trigger an evaluation for enteric hyperoxaluria or primary hyperoxaluria.

Hypocitraturia is defined as a citrate below 325mg/day. Hypokalemia can trigger hypocitraturia (along with metabolic acidosis) so be careful when prescribing a thiazide for hypercalciuria, the resulting hypokalemia could surpress citrate and increase rather than lower the risk of developing a kidney stone.

While bowel disease is usually associated with calcium oxalate stones, patients often have decreased urine pH which predisposes them to uric acid stones.

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