Introduction:
Citrate is well known crystallization inhibitor of calcium-oxalate and calcium-phosphate salts in urine. Thus, decreased urinary citrate excretion is an important risk factor for urolithiasis. However, there is a lack of widely accepted normal values of urinary citrate excretion in children and adolescents.

Objective:
The objectives of the study were to assess 24-h urinary citrate excretion and to establish its reference values in healthy pre-school- and school-age children.

Material and Methods:
The study comprised 211 healthy children aged 4–18 years (mean 10.9 ± 3.9 years); including 105 boys and 106 girls in age-matched groups. Urinary citrate concentration was measured enzymatically. 24-hour citrate excretion was expressed per kg of body weight, per m2 of standard body surface area and as ratio over urinary creatinine concentration.

Results:
A statistically significant negative correlation was noted between age and urinary citrate excretion expressed per kg of body weight or citrate/creatinine ratio. On contrary, urinary citrate excretion expressed per m2 of standard body surface area did not correlate with age. Independently of a calculation method, urinary citrate excretion was significantly higher in girls than in boys. The lowest normal urinary citrate excretion (10th percentile) for girls and boys were 1.32 mmol/1.73 m2/24h and 0.92 mmol/1.73 m2/24h, respectively.

Conclusions:
24-hour urinary citrate excretion expressed per m2 of standard body surface area in children aged 4–18 years is constant; however, due to gender differences, reference values were established separately for boys and girls. We hope, that our results will help evaluate pediatric patients with urolithiasis.