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.