Frequency of cystinuria among stone-forming patients in region of Brazil.

Occasional urine samples from 200 stone-forming individuals were screened by the successive application of the cyanide-nitroprusside test, qualitative-semiquantitative thin-layer amino acid chromatography, and quantitative ion-exchange amino acid analysis to determine the frequency of cystinuria in this region of Brazil. Only 1 homozygous cystinuria patient was detected, a lower frequency than 1 to 6 per cent reported in other countries. The patient's family showed a I/I genotype. Since 6 heterozygotes for cystinuria +/II or +/III were also detected, the relative rarity of homozygotes in this sample supports the view of the relatively greater contribution of etiologic factors other than gene frequency to stone formation. The importance of diagnosis based on quantitative amino acid analysis is emphasized because of the different therapeutic and prognostic implications of the homozygote and heterozygote forms of the disease.