**6. Biochemistry and urine analysis**

Confirmation of significantly elevated urinary cystine levels is key to establishing a diagnosis of cystinuria. The cyanide-nitroprusside test is a qualitative test traditionally used as a screening test for cystinuria; a positive result occurs when the urine turns red after the addition of the reagent, indicating a urinary cystine level >75 mg/g creatinine [2]. However, as the cyanidenitroprusside test is designed to detect amino acids containing a free sulfhydryl or disulphide bond there is the possibility of obtaining a false positive result in cases of homocystinuria and acetonuria [4]. Precise quantitative measurement of urinary cystine levels is therefore always indicated in cystinuria patients, and homozygotes will often have grossly elevated levels of >300–400 mg/L, compared to the normal level of 30 mg/L [4]. In addition to measuring urinary cystine levels by mass spectrometry, urine microscopy may also be performed to look for the hexagonal colourless crystals which are pathognomonic for cystinuria [4].
