The purpose of this research was to develop amino acid and acylcarnitine reference intervals in neonates of different gestational ages and birth weight.

   Concentrations of 11 amino acids, 31 acylcarnitines and succinylacetone were measured by tandem mass spectrometry (MS/MS) in neonates born in Moscow between 2021 and 2022. The analysis of the amino acids and acylcarnitines spectra was performed on the 4th day of life in neonates born after 37 weeks’ gestation and on days 7 and 14 in neonates born before 37 weeks’ gestation. All neonates were divided into 5 groups, depending on the gestational age at the time of birth and on the birth weights. The 0.5th -99.5th percentiles for each biomarker were compared to assess the significance of the obtained results and thus to develop reference intervals. 226225 neonate blood samples were analyzed. When assessing the difference in the amino acid and acylcarnitine profiles between different groups of neonates, the most significant differences were discovered in the concentration of arginine, leucine, methionine, phenylalanine, tyrosine, valine and proline. The concentration of these amino acids was significantly higher in the groups with low body weight and shorter gestational age at birth. When comparing the concentrations of acylcarnitines, an increase in the concentrations of C0, C4, C5, C6, C8 and C10 was highlighted in the newborns with a shorter gestational age and lower birth weight in comparison to the term neonates. Thus, this research demonstrated the complexity of interpreting the TMS data obtained from preterm neonates as well as the need in consideration of the gestational age in the analysis of neonatal screening. The research highlighted the importance of reference intervals for neonates born before 37 weeks’ gestation.