Conversion of a qualitative screening test to a quantitative measurement of urinary cystine and homocystine

Qualitative urinary screening procedures were converted to quantitative methods for urinary cystine and homocystine based on the reactions between these amino acids and cyanide-nitroprusside reagents. Cystine and homocystine are quantified by the measurement of absorbances at 521 and 524 nm, respectively. Cyanide-nitroprusside reacts with both cystine and homocystine. However, in the presence of silver nitrate, only homocystine reacts to produce a magenta color. Following the cyanide-nitroprusside reaction, absorbance must be read within three minutes for cystine and immediately for homocystine. The stability of the absorption spectra has no apparent effect on these quantitative assays. Amino acid concentrations are expressed as ratios to creatinine, which tends to eliminate false negative results in dilute urine specimens. The normal urine value for cystine and homocystine combined is 66.8 +/- 52 (n = 50) mg per g creatinine. The normal value for homocystine alone is 29.9 +/- 16.8 (n = 24) mg per g creatinine. The simplicity of these procedures allows these quantitative methods to be used as screening tests for cystinuria and homocystinuria.