Fractional excretion of uric acid in infancy and childhood. Index of tubular maturation.

Normal newborns had a low glomerular filtration rate measured by creatinine clearance which progressively increased to reach adult levels by one year of age when corrected for surface area. There was also an increased fractional excretion of uric acid (34·6% ± 11·2 SD). Progressive maturation of this tubular function was observed within the first year of life. Thereafter, stable levels approaching adult levels of excretion were maintained. Linear regression functions of log-transformed values of fractional excretion of uric acid by weight were fitted separately to the newborns, infants <1 year, and children aged 1 to 7 years. The 3 groups were found to constitute three distinct populations with regard to both the slope of the regression lines and the scatter of values about the line, which decreased significantly from the youngest to the oldest group. In low birthweight infants both glomerular and tubular maturation, as evidenced by these parameters, were related to age rather than to weight. It is suggested that the large urinary uric acid load excreted in the first days of life is facilitated by the concomitant deficiency of acidification and concentration of urine.