2256 CLINICAL VALIDATION OF A NOVEL ASSAY USED FOR MONITORING TREATMENT OF PATIENTS WITH CYSTINURIA

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characterize contemporary cohort of patients with TZ cystinuria and compare them with a concurrent cohort of homozygous MZ cystinuria. METHODS: A retrospective review of prospectively collected data was performed for 42 consecutive patients presenting with positive cyanide-nitroprasside test for cystinuria between September 2009 and September 2011. Demographic and clinical data were collected together with metabolic stone work-up, including two 24-hour urine collections with quantitative 24-hour urinary cystine excretions. RESULTS: Of the 42 patients, 36 (85.7%) and 6 (14.3%) were found to have TZ and MZ cystinuria, respectively. TZ patients were significantly older than MZ patients at first stone episode [46 (6-67) vs. 18.8 (14-44) years, p 0.01]. As expected, the median 24-hr urinary cystine excretion was significantly higher in the MZ group [1085 (6901773) vs. 392 (1461984) umol; p 0.01). Suboptimal 24-hr collection was detected in 25 (69.4%) of TZ patients and 5 (83.3%) of MZ patients with median (range) volumes of 1425 (7003400) mL and 2050 (1000-3000)mL respectively. Hyperurecemia was significantly higher in MZ group (16.7% vs. 66.7%, p 0.02). Stone composition was significantly different in both groups. Whereas 83.3% of MZ group formed pure cystine stones, 47.2%, 19.4% and 8.3% of the TZ patients formed calcium oxalate, uric acid, and cystine stones, respectively (p 0.004). Interestingly, 12 (33.3%) of TZ patients had false negative results on the cyanide-nitroprasside test. CONCLUSIONS: There were significant differences between the heterozygous (TZ) and homozygous (MZ) cystinuria patients in terms of age at first stone episode, male to female ratio, incidence of hyperuricemia and urinary stone compositions. The clinical significance remains to be elucidated.