Pediatr Nephrol (2007) 22:514–520 517 Fig. 2 The spontaneous crystallization of CaOx in urine of 1,050 children aged 3.00–17.99 years. The x-axis shows amount of ammonium oxalate (Ox2−) in mmol necessary for the onset of spontaneous crystallization. The y-axis shows the concentration of calcium ions [Ca2+] before adding Ox2−. The extremes of the hyperbola represent minimum and maximum values. The minimum hyperbola is defined by the equation [Ca2+]=0.27/(Ox2−), and the maximum is [Ca2+]=1.67/(Ox2−). The BRI values for the 5th to the 95th percentile are found between the hyperbolas [Ca2+]=0.39/(Ox2−) and [Ca2+]=0.94/(Ox2−), median [Ca2+]=0.65/(Ox2−) children 3–9 years of age, the lowest values of BRI were found in the youngest children, i.e. those 3 years old (median 0.18 1/L), whilst the highest values were found in 9-year-old children (median 0.34 1/L). However, statistical analyses did not show a difference between the values across the various age groups. The lowest median BRI value was 0.03 1/L in 9-year old children, whilst the highest median value was 2.48 1/L in 4-year-old children. The BRI values for younger children corresponding to the 5th percentile ranged from 0.03–0.08 1/L, and the 95th percentile from 1.38–2.27 1/L. In older children and adolescents aged 10–18 years, the lowest BRI values were found in the participants who were 12 years old (median 0.17 1/L), whilst the highest were in 17-year-old adolescents (median 0.31 1/L) (Fig. 3). How- ever, statistical analysis did not show significant differences between the age subgroups. The minimum value of BRI was 0.02 1/L, and was found in 11-year-old children, whereas the maximum value was 3.1 1/L in children aged 10 and 17 years. The BRI values for older children corresponding to the 5th percentile ranged between 0.05 and 0.13 1/L, whilst the 95th percentile was from 1.69–2.66 1/L. The crystallization values of CaOx, based on BRI in healthy children aged 3–18, did not exceed 2.66 1/L. No significant differences were found in the BRI values between boys and girls in either age group. Table 1 presents the BRI and BRI related to the 1.73 m2 body surface area and body mass (kg). The median BRI in relation to body surface area (1/L × 1.73 m2) was 0.39 1/L × 1.73 m2, with 5th and 95th percentile medians of 0.09 and 3.01 1/L × 1.73 m2, respectively. However, the median BRI in relation to 1 kg of weight was 0.008 1/L × kg with corresponding 5th and 95th percentiles of 0.0015 and 0.06 1/L × kg, respectively. A weak positive correlation was found between BRI and BRI/1.73 m2 (R = 0.18, P < 0.05), and a negative correlation was found between BRI/1.73 m2 and BRI/kg (R = −0.86, P < 0.05). No differences were found