Ask us: How common is it for homozygous cystinurics to not form a stone?

In our “Ask a kidney stone doctor” section, we field questions from stone formers or their family members. Today’s question is about cystinuria. See other questions and answers or ask your own question here.


October 11, 2011

Question from the UK:
“How common is it for a patient with homozygous cystinurea to never form a stone?”


Cystinuria is a rare cause of kidney stones overall, accounting for 1-2% or less of all stones. In children however, it accounts for up to 6-8% of stones. It is caused by a defect in the transport of amino acids across the cellular membrane of the renal proximal tubule and small intestine. Cystinuria is an inherited disorder.

Because of the relative rarity of cystinuria, there is limited reliable, long term data on the risk of stones. A review of the literature found the following:

Most but not all patients with homozygous cystinuria (carrying two copies of the abnormal gene) appear to develop stones throughout their lifetime. A study by the International Cystinuria Consortium reported that only 6% of individuals with homozygous cystinuria did not produce stones. The age at which individuals first formed stones also varied, with many experiencing stones between 11 and 20 years of age but with some forming stones before the age of 3.

Complicating matters, cystinuria is classified into types A, B, and AB, depending on the specific combination of two known inheritable genes (SLC3A1 and SLC7A9) that can cause the disease. The risk and age of onset of developing stones varies with which type an individual has.

Pathophysiology and Treatment of Cystinuria. Chillaron et al. Nature Reviews Nephrology. 2010.
Cystinuria Subtype and the Risk of Nephrolithiasis. Goodyer et al. Kidney International. 1998.


Before making any medical decisions, always consult with your treating physician.

About Dr. Mike Nguyen

Mike M Nguyen, MD, MPH, is a urologist and an Associate Professor of Clinical Urology at the Keck School of Medicine of USC in Los Angeles, CA. He specializes in the treatment of kidney stones with both surgery and dietary prevention and the in the treatment of kidney and prostate cancer using the latest robotic surgical approaches. He sees patients at clinics located in Los Angeles, Pasadena, and La Canada, CA. He is the founder of the website.
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4 Responses to Ask us: How common is it for homozygous cystinurics to not form a stone?

  1. Deirdre says:

    Does chronic use of Potassium Citrate (to keep the pH alkaline) have any long term drawbacks?

    • Dr. Mike Nguyen says:

      Long term potassium citrate therapy is generally well tolerated and safe but can carry some potential side effects and risks. Some patients may experience gastrointestinal upset with the medication. Taking it with meals can help alleviate this. There also appears to be conflicting evidence about whether potassium citrate can increase the risk of calcium phosphate stones by causing the urine to be too alkaline. If there is an increased risk of calcium phosphate stones, it appears to be small. Finally, potassium citrate therapy can cause an abnormally high blood potassium level in certain situations. According to the manufacturer of Urocit-K, “Conditions that may put you at risk include kidney failure, uncontrolled diabetes, severe dehydration, strenuous physical exercise if you’re unconditioned, extensive tissue breakdown, or adrenal insufficiency”.
      References: Campbell-Walsh Urology, 10th ed. and Mission Pharmacal prescribing information

  2. Deirdre says:

    Thank you for your reply. I’m 43 years old and haven’t ever formed a stone – I imagine that places me in the lucky 6% who don’t. Does this ever change for this group where stone formation begins as one grows older? Or does it remain unlikely?

    • Dr. Mike Nguyen says:

      At 43, ff you have not yet formed a stone, it is more likely that you will remain stone free. Most studies report early stone development in cystinurics as occurring before age 10 and later development being in the teens and twenties.

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