Calculi are radiopaque due to their sulphur content. In about 10% of calculi produced by patients with cystinuria there is no cystine component to the stones. Hexagonal cystine crystals are easily identified in the urine and the typical amino acid excretory pattern confirmed by chromatography.
The cyanide-nitroprusside urine test is positive in cystinuria and in homocystinuria.
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