Cystinuria

Key points

• Cystinuria is an inherited disorder causing recurrent cystine stones because renal tubular transport of cystine, ornithine, lysine and arginine (COLA) is impaired.
• SLC3A1 is on chromosome 2 and SLC7A9 is on chromosome 19; cystinuria is genetically heterogeneous and is inherited in an autosomal-recessive pattern overall.
• Recurrent renal stones , often beginning at a young age, are characteristic.
• Cystine stones are typically poorly radiopaque/semi-opaque ; they may have a yellow or crystalline appearance, but colour is not a reliable diagnostic criterion.
• Cyanide-nitroprusside testing can screen for urinary cystine, but confirmatory evaluation uses quantitative urinary cystine/stone analysis and appropriate specialist assessment.
• Management: maintain high urine volume, alkalinise urine to pH >7.5 and use a thiol-binding agent such as tiopronin for recurrent/high-risk disease despite conservative measures. The duplicate original Cystinuria paragraph is not repeated.
• MRCP discriminator: young patient + recurrent stones + hexagonal crystals + poorly radiopaque stone → cystinuria.
• Key treatment target: urine volume >3 L/day where tolerated and urine pH >7.5; tiopronin is used when recurrent/high-risk disease persists.