As the levels of cystine in the urine increase, it forms cystine crystals, resulting in kidney stones. Cystine crystals form hexagonal-shaped crystals that can be viewed upon microscopic analysis of the urine. The other amino acids that are not reabsorbed do not create crystals in urine.The overall prevalence of cystinuria is approximately 1 in 7,000 neonates (from 1 in 2,500 neonates in Libyan Jews to 1 in 100,000 among Swedes). Pathophysiology
Cystinuria is characterized by the inadequate reabsorption of cystine in the proximal convoluted tubules after the filtering of the amino acids by the kidneys glomeruli, thus resulting in an excessive concentration of this amino acid in the urine. Cystine may precipitate out of the urine, if the urine is neutral or acidic, and form crystals or stones in the kidneys, ureters, or bladder. It is one of several inborn errors of metabolism included in the Garrods tetrad. The disease is attributed to deficiency in transport and metabolism of amino acids. Diagnosis
Blood: Routine hemogram along with blood sugar, urea, and creatinine. Urine: For cystine crystals, and casts. The most specific test is the cyanide–nitroprusside test
Ultrasound/CT scan to reveal if a stone is present. Genetic analysis to determine which mutation associated with the disease may be present. Currently genotyping is not available in the United States but might be available in Spain, Italy, UK, Germany and Russia (by private companies in Germany and Russia).Regular X-rays often fail to show the cystine stones, however they can be visualized in the diagnostic procedure  that is called intravenous pyelogram (IVP).
Stones may show up on XR with a fuzzy gray appearance. They are radioopaque due to sulfur content, though more difficult to visualize than calcium oxalate stones. Treatment
Initial treatment is with adequate hydration, alkalization of the urine with citrate supplementation or acetazolamide, and dietary modification to reduce salt and protein intake (especially methionine). If this fails then patients are usually started on chelation therapy with an agent such as penicillamine. Tiopronin is another agent. Once renal stones have formed, however, the first-line treatment is endoscopic laser lithotripsy. ESWL (Extracorporeal shock wave lithotripsy) is often not effective because of the hardness of the stones that do not fragment easily. Conventional open-abdominal surgery is rarely used but has proven to be effective treatment modalities for patients with more advanced disease. Adequate hydration is the foremost aim of treatment to prevent cystine stones. The goal is to increase the urine volume because the concentration of cystine in the urine is reduced which prevents cystine from precipitating from the urine and forming stones. People with cystine stones should consume 5 to 7 liters a day. The rationale behind alkalizing the urine is that cystine tends to stay in solution and causes no harm. In order to alkalize the urine, sodium bicarbonate has been used. One must be careful in alkalizing their urine because it could lead to other forms of stones in process of preventing cystine stones. Penicillamine is a drug that acts to form a complex with cystine that is 50 times more soluble than cystine itself.