In addition to spot urine and serum tests, a formal 24-h urine collection is essential to the determination of stone-forming risk. Because of variation in diet and fluid intake, results from two separate 24-h urine collections, ideally six weeks after the patient achieves a stone-free status, should be used to guide treatment. To ensure that there is a complete 24-h collection of urine, a total creatinine should be greater than 15-20 mg/kg. When interpreting 24-h urine results, it is important to remember that adult reference values are not necessarily applicable to the pediatric population.[45
] Borawski et al
., measured standard urinary risk factors in 46 healthy children without a history of stones.[46
] After adjusting for urinary creatinine and body weight, multiple metabolic parameters including oxalate, uric acid, citrate, magnesium, sodium, phosphorus and potassium significantly decreased with increasing age. lists several standard urinary parameters with known pediatric reference ranges.[45
] In addition to the concentrations of individual components, the supersaturation indices are especially useful to quantify crystallization potential.[47
Adult and pediatric reference values for 24-hour urinalysis
Recent investigations into stone-forming risk have resulted in the Bonn risk index (BRI) which may better predict recurrent calcium oxalate stone formation.[48
] The BRI is the ratio of ionized urinary calcium to the amount of ammonium oxalate required to induce calcium oxalate crystallization in 200 ml of urine. This ratio remains relatively stable in children across age and sex. BRI values in children with renal stones are 15-fold higher when compared to healthy children. Future research on the BRI is required to define its potential as a predictor of stone formation in asymptomatic children.
Perhaps the single most significant finding on a 24-h urine collection is the total volume. Pediatric stone-formers tend to have vastly insufficient oral fluid intake, and this may be the single most important factor in stone formation and recurrence. Low volumes documented on a 24-h urine (<1 mL/kg/hour) may serve as evidence of poor intake and may facilitate a discussion with families of the need to aggressively increase fluid intake throughout the day.