Determination of urinary cystine for monitoring anticystinuric therapy

Cystine is assayed in urine by a method adapted from the procedure of Haux and Natelson (Clin Chem 1970;16:366–369) and modifications of the method, which improved its specificity, are presented. Assay imprecision (CV) at four different cystine concentrations (range: 0.042–1.658 mmol/l) varied from 23.1% to 3.7%. Analytical recovery was 94.6 ± 6.0%. d-Penicillamine, α-mercaptopropionylglycine, l-glutamine and metabolites, added to urine, had negligible effect on the results of the cystine assay. Samples were stable for 3 wk when stored at 4°C or frozen. Cystine excretion (mmol/24 h) in apparently healthy persons ranged from 0.041 to 0.170 ( mean ± sd : 0.099 ± 0.039 ). Cystine excretions are presented in patients treated with anticystinuric drugs.