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Purpose: Urinary stone disease is becoming more common in children. We have previously reported a high rate of urinary metabolic abnormalities in stone-forming children including hypercalciuria and hypocitraturia. At our institution, 24 hour urinary metabolic evaluations are typically obtained after the first stone episode. The purpose of this study is to determine if a Ca/citrate ratio can help determine which children are most at risk for recurrent stone formation and who may benefit from more early, aggressive medical intervention.
Material and Methods: A retrospective cohort study was performed to assess calcium-to-citrate ratios in children with urolithiasis. Two consecutive day 24-hour urine collections were performed and evaluated at an outside central laboratory. The 24-hour urinary excretions of calcium and citrate were analyzed, and the Ca/citrate ratio was calculated. The average of the two collections was used for comparison. The patients were stratified into solitary or recurrent stone formers by review of the medical record blinded to the analysis of their urinary indices. Patients with multiple stones on presentation were included in the recurrent stone-forming group. A control group of normal patients with no stone disease was used for comparison. Univariate analysis between means was performed with a two-tailed t-test. A subgroup analysis of patients with hypercalciuria was also performed.
Results: A total of 74 solitary stone formers and 98 recurrent stone formers were identified. The mean age was 14 years and the mean weight was 46 kg. Age and gender were well matched between the two study groups. 96 normal children served as a control group. The majority of known stones were calcium oxalate, and there were no radiolucent stones in those with unknown composition. The mean calcium-to-citrate ratio in solitary stone formers was 0.409 and in recurrent stone formers was 0.750 (p = 0.02). The mean value in normal children (0.412) was significantly lower than in recurrent stone formers (p=0.03) but similar to solitary stone forming children (p=0.97). In children with hypercalciuria, the mean Ca/citrate ratio was 0.546 in solitary stone formers and 0.821 in those with recurrent stones (p=0.24). Those with hypercalciuria and recurrent stones had a significantly higher mean Ca/citrate ratio than normal controls (p=0.02). Children with hypercalciuria and solitary stones also had a higher ratio than controls (p=0.02).
Conclusions: There are significant differences in the urine calcium-to-citrate levels between solitary and recurrent calcium stone-forming children. Solitary stone formers have similar ratios to controls except in patients with hypercalciuria. This may allow more precise risk stratification and treatment to prevent recurrent stone episodes.