516 Fig. 1 Body mass index (BMI) in studied girls and boys aged 3–18 years compared to age and sex-matched reference range Pediatr Nephrol (2007) 22:514–520 in urine to the amount of added ammonium oxalate (Ox2−) necessary for the spontaneous crystallization of CaOx. BRI in healthy children ranged between 0.06 and 1.93 1/L. The values between the 5th and 95th percentiles are found between the two borderline arrows. This diagram also presents the minimum and maximum concentrations of [Ca2+] and the required amount of added oxalate (Ox2−). The concentration of [Ca2+] in urine ranged from 0.25 mmol/L (5th percentile) to 0.89 mmol/L (95th percentile) with a median of 0.42 mmol/L, and an amount of added (Ox2−) ranging from 0.46 mmol (5th percentile) to 3.53 mmol (95th percentile) with a median of 1.65 mmol. Figure 3 presents the BRI values in children representing various age groups. We considering the subjects in two main subgroups, younger children aged 3–9 years, and older children and adolescents aged 10–18 years. In