Abstract
Urinary excretions of guanidinoacetic acid (GAA), β2-microglobulin (β2MG), and N-acetyl -β-D-glucosaminidase (NAG) are putative parameters of proximal tubular functions. In this study, I investigated the effect of age on these parameters in newborn infants. Urinary samples (N = 97) from 25 fullterm infants and 6 preterm infants without serious complications were studied. Urinary concentrations of GAA, β2MG, and NAG activity, as well as creatinine (Cr), were measured in each sample. The ratio of urinary GAA to Cr (U-GAA/Cr ; mg/g) in the neonates ranged from 4.6 to 440.0 mg/g (mean±SD, 111.76±78.90 mg/g). U-GAA/Cr in healthy adults was 67.1±32.1 mg/g. U-GAA/Cr had a positive correlation with postnatal age (r =0.33, p<0.001) and with postconceptional age (r =0.43, p<0.001).
In contrast, the ratios of urinary β2MG to Cr (U-β2MG/Cr ; mg/g) and U-NAG/Cr (U/g) had negative correlations with postconceptional age, but not with postnatal age. U-GAA/Cr correlated with neither U-β2MG/Cr nor with U-NAG/Cr.
We speculate that U-GAA/Cr might reflect proximal tubular function, in a manner different from that of urinary β2MG and urinary NAG activity.
