Changes in the urinary excretion of creatinine, albumin and N-acetyl-beta-D-glucosaminidase with increasing age and maturity in healthy schoolchildren

Raised urinary levels of albumin and N-acetyl-beta-D-glucosaminidase (NAG) are predictive of abnormal renal function and excretion of these substances is often expressed as a creatinine ratio. However, it is important to establish normal reference limits of albumin and NAG excretion for comparison of values from patients. For this reason, overnight excretion rates of creatinine, albumin and NAG were determined in timed overnight urine samples from 528 healthy schoolchildren (260 boys, 268 girls; 4-16 years) of normal size. There was a significant correlation with age and puberty for all substances in both sexes (P < 0.01). Peak creatinine excretion occurred at 16 years in boys, at 15 years in girls and at pubertal stage 5 in both sexes. Maximum albumin excretion was seen at 15 years and genital stage 5 in boys and at 16 years and breast stage 4 in girls. Peak NAG excretion occurred earlier, at 14 years and genital stage 4 in boys and at 13 years and breast stage 3 in girls. Boys excreted significantly more creatinine compared with girls before and during puberty (reflecting greater muscle mass) (P < 0.01) while excretion rates for albumin and NAG were similiar in both sexes. Height and weight combined accounted for 58% and 29% of the variation in creatinine and NAG excretion respectively, while height alone predicted 20% of variation in albumin excretion. Conclusion Age and puberty influence the urinary excretion of albumin and NAG while sex has an additional effect on creatinine excretion. The urinary excretion of albumin and NAG in children with renal disorders should be compared with age-related normal ranges while creatinine excretion could be used as a marker of muscle growth.