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Cystinuria is an inborn error of amino acid transport in the kidneys and intestine that allows excessive urinary excretion of cystine and other dibasic amino acids. This results in recurrent cystine renal calculus formation.

Cystinuria occurs in about 1 in 15,000 live births. It's the most common defect of amino acid transport. Onset of symptoms commonly occurs between ages 10 and 30 (although it can occur in infants as young as age 1 or in adults well into their 30s). With proper treatment, the prognosis is good.


Cystinuria is an inherited autosomal recessive disorder. The kidneys don't adequately resorb certain amino acids during the filtering process, resulting in excess excretion of these amino acids. The amino acids may precipitate (solids separate from liquids) and form crystals or stones in the kidneys, ureters, or bladder.

The disorder is usually diagnosed after an episode of stones, where analysis of the composition of the stones indicates cystine. Less than 3% of known urinary tract stones are cystine stones.

Signs and Symptoms

The stones that form in cystinuria are directly or indirectly responsible for all of the signs and symptoms of the disease, including:

  • Hematuria - blood in the urine;
  • Flank pain - pain in the side, due to kidney pain;
  • Renal colic - intense, cramping pain due to stones in the urinary tract;
  • Obstructive uropathy - urinary tract disease due to obstruction; and
  • Urinary tract infections.

Diagnostic tests

Various diagnostic tests confirm cystinuria. For example, chemical analysis of calculi shows cystine crystals with a variable amount of calcium. Pure cystine calculi are radiolucent on X-ray, but most contain some calcium. These calculi are light yellow or brownish yellow and granular and may be large.

Blood studies may show an elevated white blood cell count, especially if the patient has a urinary tract infection (UTI), and elevated clearance of cystine, lysine, arginine, and ornithine.

Urinalysis with amino acid chromatography indicates aminoaciduria, consisting of cystine, lysine, arginine, and ornithine. Urine pH normally is less than 5.0.

Microscopic examination of urine shows hexagonal, flat cystine crystals. When glacial acetic acid is added to chilled urine, cystine crystals resemble benzene rings.

An oxide-nitroprusside test result is positive. In cystinuria, a urine specimen made alkaline by adding ammonia turns magenta when nitroprusside is added.

Confirming tests also include excretory urography to determine renal function and kidney-ureterbladder radiography to detect the size and location of calculi. Because cystine calculi are translucent, these tests can only confirm a diagnosis if other minerals are deposited on the calculus or if an obstruction is present.


No effective treatment exists to decrease cystine excretion. Increasing fluid intake to maintain a minimum 24-hour urine volume of 3,000 ml and reduce drine cystine concentration is the primary means of diluting excess cystine and preventing cystine calculus formation.

Sodium bicarbonate and an alkaline-ash diet (high in vegetables and fruit and low in protein) alkalinize urine, thereby increasing cystine solubility. However, this therapy may provide a favorable environment for formation of calcium phosphate calculi.

Penicillamine also can increase cystine solubility, but it should be used with caution because of its toxicity and the high incidence of allergic reactions.

Dissolving cystine calculi may take up to 12 months (in 40% of patients). Irrigating agents, such as tromethamine or acetylcysteine, have been effective. Electrohydraulic ultrasonography may be used to break the calculi into fragments that can be passed.

Treatment also may include surgical removal of calculi, when necessary, and appropriate measures to prevent and treat UTI.


There is no known prevention for cystinuria. Any person with a known history of stones in the urinary tract should drink enough fluids to maintain a high urinary output by day and by night, allowing stones and crystals to be excreted before they become large enough to cause symptoms. Keeping the urine alkaline diminishes the risk of cystine stones.

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