Urinary creatinine in very low gestational age neonates: importance for interpretation of creatinine-normalized urinary amino acids

OBJECTIVES: Urinary creatinine concentration is widely used to normalize urinary analytes. However, its reliability in preterm neonates remains uncertain. This study aimed to evaluate urinary creatinine concentrations in very low gestational age (VLGA) neonates and assess its impact on the interpretation of urinary creatinine normalized urinary amino acids. METHODS: Spot urine samples were collected from VLGA neonates (22-31 weeks gestational age, n = 32) and full-term neonates (≥ 37 weeks, n = 22) during the first days of life. Urinary creatinine was measured serially, and biological variation (within- and between-subject) was assessed. Urinary amino acids were measured on the 4th day in full-term neonates and in VLGA neonates at term-equivalent age using LC-MS/MS and expressed both as absolute concentrations (µmol/l) and normalized to creatinine (mmol/mol creatinine). RESULTS: Urinary creatinine concentrations were significantly lower in VLGA neonates compared to full-term neonates during the first days of life (p ≤ 0.004). A positive correlation between birth weight and urinary creatinine was observed (r = 0.46, p = 0.002). Within-subject biological variation was lower in VLGA neonates, while between-subject variation was comparable between groups. Urinary amino acid concentrations expressed in μmol/l did not differ significantly between groups, however, when expressed relative to creatinine, multiple significant differences emerged. CONCLUSIONS: Gestational age should be considered when interpreting creatinine normalized urinary amino acids concentrations in neonates.