Urinary reference values for stone risk factors in children

Purpose: Reference values for stone risk factors in 24-hour urine samples for nonstone forming children are limited. We measured urinary stone risk factors in healthy children 3 to 18 years old, and sought to determine whether the risk factors are affected by age. Materials and Methods: A total of 48 healthy subjects with no history of stone disease, endocrine abnormalities or urological surgery were recruited from the Naval Medical Center in San Diego. Subjects were then further divided into 4 age groups, each separated by 5 years. A single outpatient 24-hour urine sample was obtained and analyzed. Urine chemistries were adjusted for urinary creatinine and body weight. Results: After excluding under collected samples 46 urine samples were analyzed. Urinary pH and volume decreased with increasing age, although the difference in pH did not reach statistical significance. Unadjusted urinary parameters failed to show statistical difference among the age groups. When adjusted for urinary creatinine and body weight all urinary parameters (calcium, oxalate, uric acid, citrate, magnesium, sodium, phosphorus and potassium) decreased with increasing age (statistically significant except for calcium). Conclusions: Stone risk factors in 24-hour urine samples decrease with increasing age in healthy, nonstone forming children. Normative data, derived by adjustment with urinary creatinine or body weight and stratified according to quintiles of age; should be useful in defining abnormal stone risk factors in children with stones.