Objective:The blood free carnitine levels of preterm infants with neonatal respiratory distress syndrome (NRDS) were detected within 7 days after birth, and the correlation between blood free carnitine levels and NRDS in preterm infants was explored.Methods:Seventy premature infants with gestational ages from 28 to 36 weeks who were admitted to the NICU of the Affiliated Hospital of Inner Mongolia Medical University from January 1, 2017 to December 31, 2019 were selected as the participants.Among them, 35 cases were in the NRDS group, while 35 cases of premature infants without NRDS were chosen as the control group.Heel blood samples were collected from all subjects within 6 hours, 3 days and 7 days after birth, and the levels of blood free carnitine were detected by tandem mass spectrometry.Results:Within 7 days after birth, the levels of free carnitine in blood of premature infants in the two groups gradually decreased with time, but the decreasing trend was not similar( F=4.096, P=0.021). Compared with 6 hours after birth, the blood free carnitine level in NRDS group decreased significantly with 3 days after birth[(35.24±9.58) μmol/L vs.(23.96±7.12) μmol/L, P<0.05]. The levels of blood free carnitine in NRDS group at 6 hours and 3 days after birth were significantly lower than those in control group[(35.24±9.58) μmol/L vs.(40.85±11.39) μmol/L; (23.96±7.12) μmol/L vs.(29.60±8.05) μmol/L, P<0.05]. There was no significant difference in blood free carnitine levels between the two groups at 7 days after birth ( P>0.05). Conclusion:The blood free carnitine levels of premature infants decreased within 7 days after birth.The decrease of carnitine level in premature infants with NRDS may be related to the occurrence and development of NRDS.