A 20-year-old man with history of recurrent kidney stones is seen in a nephrology clinic. He was first found to have a 2 cm stone in the right kidney at age 8 that caused hydronephrosis requiring nephrostomy tube placement and stone extraction. A 24-hour urine collection showed very high urine cystine excretion (more than 400 mg/day) confirming the diagnosis of cystinuria. Since then, he has been maintained on potassium citrate to alkalize his urine. He reports that he drinks at least 3 liters of water a day and does his best to limit dietary sodium and protein intake.

Today, he feels well and denies any flank pain, gross hematuria, or passing any stones recently. Surveillance kidney ultrasound scans, however, revealed new stones, 1 in each kidney measuring 0.5 cm in diameter. Current medications include potassium citrate 15 mEq three times a day and vitamin D 2000 units daily.

Laboratory studies results:

  • Serum creatinine 0.96 mg/dL
  • Blood urea nitrogen 9 mg/dL
  • Serum sodium 139 mmol/L, potassium 3.9 mg/dL, chloride 99 mmol/L, CO2 31 mmol/L, calcium 9.6 mg/dL
  • Uric acid 5.7 mg/dL
  • Urine pH 8.0, SG 1.004

24-hour urine collection results:

  • Volume 3.3 liter
  • Sodium 90 mg/day
  • Calcium 175 mg/day
  • Oxalate 37 mg/day
  • Citrate 800 mg/day
  • Uric acid 0.6 g/day
  • Urine cystine concentration 743 mg/L (total 2451.9 mg/day)
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