Analysis of 24-hour urine parameters as it relates to age of onset of cystine stone formation.

[cystinuria]

Cystinuria is an inherited disorder characterized by the impaired reabsorption of the dibasic amino acid , cystine , in the proximal tubule of the nephron . Cystine stones most frequently occur in the first through third decades of life with a decreased incidence in old age . We hypothesize that patients presenting with first stone event at an older age would have more favorable 24 -hour urine parameters compared with those patients who present early .This is a retrospective review of 94 patients with cystinuria and cystine stones who underwent a 24 -hour urine test . The patients were divided into four groups based on age at first clinical stone event : group 1 , children aged <18 years (n = 37 ); group 2 , 18 to 32 years (n = 19 ); group 3 , 33 to 50 years (n = 19 ); and group 4 , >50 years (n = 19 ). All 24 -hour urines were collected prior to any medical therapy . Statistical analysis was performed using analysis of variance and Tukey pairwise comparison test .There was a significant increase in cystine supersaturation (SS ) for the young adults (group 2 ) as opposed to older adults (groups 3 and 4 ) though cystine excretion was not different among the groups . A higher urine volume appeared to account for the lower cystine SS in the older adults . The children were found to have cystine SS similar to the young adults .In this retrospective study , a correlation was found between the age of onset of cystine stones and 24 -hour urine parameters . Cystinuric patients presenting with first stone event at an older age have lower cystine SS and increased urinary volume compared with patients presenting earlier , likely conferring a protective effect .