Citrate, magnesium and oxalate urinary excretion in normal children

We measured 24 hour urinary excretion of citrate (Cit), magnesium (Mg) and oxalate (Ox) in 228 normal children, 87 boys and 141 girls, aged 4-16. Urinary citrate levels varied widely and were affected by age and sex from age 10 onwards, being significantly higher in girls. However when they were expressed as mg/kg body weight per day they were not longer affected by age but the difference between the sexes persisted: boys 9.76 +/- 5.88 mg/kg/24 hr, girls 11.26 +/- 6.10 mg/kg/24 hr (p < 0.01). The citrate/calcium index (I Cit/Ca) was 5.8 +/- 10.0 mg/mg for boys and 8.2 +/- 10.1 mg/mg for girls (P < 0.05). The urinary excretion of Mg decreased with age, in both sexes and the two sexes did not differ significantly When Mg excretion was expressed per 1.73 sq m of body surface area it no longer varied with age, the mean for the whole sample being 118 +/- 39 mg/1.73 sq m/24 hr. The Mg/Ca index (I Mg/Ca) varied widely from a 3rd centile of 0.35 mg/mg to a 97 th centile of 6 mg/mg. Oxaluria did not differ between the sexes, it decreased significantly with age but when expressed per 1.73 sq m the relationship to age was lost. The mean result was 36.92 +/- 13.72 mg/1.73 sq m/24 hr. The Cit/cr, Mg/cr and Ox/cr ratios measured on a spot urine sample did not correlate well enough with 24 hour urinary excretion to make them useful substitutes for 24 hour collection.