Evaluation of the nitroprusside test for the diagnosis of cystinuria

Because of the wide acceptability of the nitroprusside test for the diagnosis of cystinuria, an analysis of 525 consecutive urine tests performed at this laboratory was undertaken, the nitroprusside test result being correlated with the urinary cystine excretion as determined by paper chromatography, high-voltage electrophoresis and chemical tests. It was found that the nitroprusside test clearly separated urine samples into categories of cystine excretion within the normal, heterozygote and homozygote range (P less than 0-001). However, 7 (1-33%) urine specimens from homozygous cystinurics gave a negative nitroprusside result. If properly performed on a concentrated urine specimen, this test is a very accurate means of diagnosing cystinuria (expected error approximately 1%), but it is not infallible.