Urinary mineral excretion in healthy Iranian children

The purpose of this study was to determine normal reference values for urinary calcium/creatinine (Ca/Cr), phosphate/creatinine (P/Cr), magnesium/creatinine (Mg/Cr), sodium/creatinine (Na/Cr), potassium/creatinine (K/Cr), sodium/potassium (Na/K), calcium/sodium (Ca/Na), and uric acid/creatinine ratios in healthy Iranian children. Nine hundred and ninety children (515 boys, 475girls) aged 1 month to 14 years were studied. Two non-fasting random urine specimens (1 week apart) from each subject and 24-h urine collections from 114 children were analyzed for Ca, P, Mg, uric acid, Na, K, and Cr. Urinary Ca/Cr, P/Cr, Mg/Cr, Na/Cr, K/Cr, Na/K, Ca/Na, and uric acid/creatinine ratios were determined from each sample. Non-fasting Ca/Cr, P/Cr, Na/Cr, K/Cr, Na/K, Ca/Na, and uric acid/creatinine ratios were not significantly different between the sexes (P>0.05). Urinary Mg/Cr ratios were higher in girls than boys (P<0.001). No significant relationships were found between urinary Ca/Cr and urinary Na/Cr, urinary Ca/Cr and urinary Na/K, and urinary Ca/Cr and urinary Ca/Na (P>0.05). The P/Cr values showed a gradual decrease with age (from mean±SD 0.962±0.172 mg/mg at 1 month of life to 0.318±0.124 mg/mg at 14 years) (P<0.05). The Ca/Cr ratio was highest between 6 months and 3 years (mean±SD=0.047±0.041 mg/mg). Following a moderate decrease it stabilized by the age of 7 years (mean±SD=0.038±0.044 mg/mg). Urinary ratios of Mg/Cr were significantly higher in children under 10 years (mean±SD=0.042±0.015 mg/mg) compared with the 11- to 14-year age group (mean±SD=0.031±0.001 mg/mg) (P<0.05). The uric acid/creatinine ratios decreased from 0.060±0.077 mg/mg in those less than 7 years to 0.041±0.033 in the 11– to 14-year group. Urinary Na/Cr ratios were significantly lower in younger age groups compared with the older age group (P<0.05). Urinary K/Cr ratio was highest in younger children, and then steadily decreased with age. There was no correlation between 24-h urinary Ca and Na excretion. The mean Ca/Na ratios significantly decreased with advancing age (P<0.05). The solute/creatinine ratios in the non-fasting urine samples correlated well with the 24-h solute excretion. We provide reference values for urinary Ca/Cr, P/Cr, Mg/Cr, Na/K, K/Cr, and uric acid/creatinine ratios in normal Iranian children. A child's age and ethnicity should be taken into consideration when assessing the urinary solute/creatinine ratios.