Correspondence between Ca(2+) and calciuria, citrate level and pH of urine in pediatric urolithiasis

BACKGROUND: Hypercalciuria and hypocitraturia are considered the most important risk factors for urolithiasis. Citrate binds to urinary calcium to form a soluble complex which decreases the availability of ionized calcium (Ca(2+)) necessary for calcium oxalate formation and phosphate crystallization. The aims of this study were to assess the Ca(2+) fraction in relation to total calciuria, citraturia and urinary pH and to determine whether urinary Ca(2+) concentration is a helpful biomarker in metabolic evaluation of children with urolithiasis. METHODS: We collected 24-h urine samples from 123 stone-forming children and adolescents with hypocitraturia and from 424 healthy controls. Total calciuria (total calcium, Ca(total)), Ca(2+), pH, citrate, oxalate and Bonn Risk Index (BRI) were assessed and compared between the two groups. RESULTS: Total calciuria and Ca(2+) content were higher in stone-formers than in the healthy children. In both stone-formers and controls, Ca(2+) content was inversely related to citraturia and urinary pH, whereas the Ca(2+)/Ca(total) ratio differed slightly between the groups. A large variability in Ca(2+) level was found across individuals in both groups. The BRI increased with increasing calciuria and urine acidity. CONCLUSIONS: Compared to controls, stone-formers with hypocitraturia demonstrated a higher urinary Ca(2+) concentration, but this was proportional to calciuria. The large individual variability in urinary Ca(2+) content limits its practical use in metabolic evaluation of children with urolithiasis. However, the Ca/Citrate ratio may be a useful clinical tool in evaluating children with urolithiasis.