Urinary hydroxyproline in infants with and without fractures/rickets

Abstract

Molar ratios of peptide-bound and free hydroxyproline:creatinine (OHPr:Cr) in urine were measured at 3, 6, 9, and 12 months of age in two groups of very-low-birthweight (VLBW ≤1500 g) infants. Group A (15 infants) had radiographically confirmed fractures and (or) rickets (F/R); Group B (17 infants) did not. The urinary peptide-bound OHPr:Cr ratio varied widely within groups and was greatest at three months in both groups: A = 0.81 ± 0.45 and B = 0.55 ± 0.32 (mean ± SD). The ratio decreased with increasing postnatal age for each group but was not statistically different between groups throughout the study. The urinary free OHPr:Cr ratio also was greatest at age three months (A = 0.32 ± 0.15 and B = 0.53 ± 0.46), rapidly decreasing afterwards, and was not statistically different between groups throughout the study. We conclude that, in VLBW infants, bone turnover as indicated by the urinary peptide-bound OHPr:Cr ratio is highest during early infancy; however, the wide range of values for this ratio suggests that its use alone is not sufficient for detection of F/R in VLBW infants. The rapid decrease in free OHPr:Cr ratio is presumably related to the maturation of renal tubular function.