Analysis of 24-Hour Urine Parameters As It Relates to Age of Onset of Cystine Stone Formation

Introduction: 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. Patients: 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. Results: 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. Discussion: 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.