The ratios of urinary β2-microglobulin and NAG to creatinine vary with age in children.

BACKGROUND: Although urinary biochemical markers can be assessed by their ratio to urinary creatinine (U-Cr) concentration, reference values in adults may not be applicable to children because the amount of Cr excreted varies by body size. We therefore measured the relationship between age and the ratios of urinary β-2-microglobulin (U-β2MG), N-acetyl-β-d-glucosaminidase (U-NAG), calcium (U-Ca) and protein (U-Pr) concentration to those of U-Cr in children.

METHODS: Fifty-seven patients aged >1 year with benign familial hematuria (median age, 6.3 years) were divided into three age groups: 1-4, 5-9, and ≥10 years. Urinary biomarkers were assayed using actual values; ratios to actual U-Cr values; and our standardized metric, namely 100-fold the ratio of serum Cr to U-Cr concentration; and the relationship of each of these to age was determined.

RESULTS: The ratios of U-β2MG, U-NAG and U-Ca to Cr varied significantly by age, being higher in younger than in older children, but the actual and standardized values of each did not vary by age, nor did any measurement of U-Pr.

CONCLUSIONS: The ratios of urinary markers of tubular function, including U-β2MG, U-NAG and U-Ca, to Cr vary by age, being higher in younger children. In contrast, the ratios of urinary markers of glomerular filtration (such as U-Pr)to Cr do not vary by age, making them suitable for corrections relative to Cr.