The aim of NAPPA is to develop new models to improve our understanding of how children process ('metabolise') penicillins, to help us discover if we can improve the dosing regimens used for children in the future. The study population will involve children and neonates who need a penicillin antibiotic as part of their normal National Health Service (NHS) care, and who also need blood tests or intravenous access. Therefore the antibiotic study can be done without the need for any extra invasive procedures, and there will be no change to the medication therapy received by participants. In addition, NAPPA will evaluate whether it is feasible to measure antibiotic levels on a tiny drop of blood put onto absorbent paper ("dried blood spot"). This allows the sample to be stored and tested later on. NAPPA will compare the level of the antibiotic using this method with the level measured in the usual way, to show if this is a reliable method that could be used routinely in the future.

Inclusion Criteria:

• Child aged under 16 years receiving one of the specified penicillins and with intravenous access or blood test(s) as part of their routine clinical care.
• Informed consent form signed by parent or legal guardian.

Exclusion Criteria:

• Any child or infant unlikely to survive 48 hours after recruitment.
• Patient known to be pregnant.
• Known allergy or hypersensitivity to beta-lactam antibiotics (including penicillins and cephalosporins) or beta-lactamase inhibitors.
• Patient receiving (or planned to receive) haemofiltration, haemodialysis, peritoneal dialysis, extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass.

Children (aged under 16 years) and infants (including both full-term and premature neonates), admitted to hospital and routinely prescribed one (or more) of the study penicillins according to local hospital policy.