An evaluation of four methods for the detection of heterozygous cystinuria

Heterozygotes for cystinuria (types II and III) may be detected on the basis of slight to moderate elevations in urinary cystine, lysine and sometimes arginine. This seems to be a common genetic trait, and there are indications of an increased risk of urinary stone disease. In order to test the sensitivity and reliability of the procedures normally used for screening and diagnosis of heterozygous cystinuria, we studied 32 heterozygotes previously diagnosed by ion-exchange chromatography of urinary amino acids, and 23 healthy individuals. A random urine sample from each subject was analysed using the cyanide-nitroprusside test, thin-layer amino acid chromatography, colorimetric estimations of cystine and lysine, and ion-exchange amino acid chromatography. Thin-layer chromatography provided the highest sensitivity. Thus the frequency of heterozygotes calculated in previous studies, based on screening by the cyanide-nitroprusside test, may be under-estimated. The colorimetric estimations of cystine and lysine provided low sensitivity as screening tests and, compared with the ion-exchange chromatography, were unreliable diagnostic methods.