The objective of NEOPOPI is to conduct a population pharmacokinetic study of metronidazole in neonates, in order to evaluate and optimize neonatal dose regimen.

There will be no change to the medication treatment received by participants. An opportunistic pharmacokinetic sampling approach will be followed: samples will be scavenged from blood or cerebrospinal fluid drawn for routine biochemical tests. In this way, no additional invasive tests will be needed.

• Administration of the antibiotic according to the usual procedures for prescribing services: in particular, neither the indications nor the doses nor the methods of administration are fixed by the protocol
• Opportunistic sampling strategy: no biological samples are specifically collected for the purposes of the study (measurements of concentrations on "bottoms" or "left-over" samples); the performance of this non-invasive sampling strategy has been previously demonstrated in the neonatal population.
• Micro-analytical method (assay of concentrations on micro-volumes, of the order of 50μL)
• Population pharmacokinetic analysis

Inclusion Criteria:

• In case of birth at gestational age ≥ 37 weeks of amenorrhea (SA): inclusion of children of postnatal age <28 days
• In case of birth at a gestational age <37 SA: inclusion of post-menstrual age children (ie gestational age + post-natal age) <44 SA 2. Benefiting from metronidazole antibiotic therapy, as part of their routine independent clinical management of the study, whether the targeted infection is suspected or proven 3. Social Security Affiliates 4. No opposition of parents to participation in the study

Non-Inclusion Critéria Treatment with metronidazole initiated before arrival in the investigative center (> 1 dose).

Exclusion Criteria:

• None

Neonates (including both preterm and full-term neonates) receiving amoxicillin as part of their routine clinical care (for suspected or proven neonatal sepsis).