A major challenge in children's infections is that it is difficult to work out which bug is making them unwell. Tests can find the bugs that are present though there can be more than one. Some bugs may just be bystanders and not actually making the child sick. Children still receive antibiotics because it is not always clear that they don't need them.

This project explores whether measuring how the body is reacting to the bugs gives precise information about which bug is actually making them sick. It will investigate children in intensive care who are suspected of having a chest infection.

This study uses a novel technology called "metagenomics" to detect any bacteria or viruses in the lung. Alongside this, investigators will measure how the lungs respond to the bugs through further tests of cells and proteins collected from the lung fluid. This fluid will be tested to see if the response is due to bacteria or viruses.

Collecting lung fluid samples requires that children are sedated and intubated, having a breathing tube in place. This means that only children intubated in intensive care are eligible. Extra samples of lung fluid and blood will be collected when being taken for routine clinical care.

If these tests work, they have the potential to give rapid and accurate information about what type of infection is taking place in the lung. This means the correct antibiotics can be given to children who need them and avoid the harms of giving them to children who do not. This can reduce cost, improve patient outcomes and help limit the development of antibiotic resistance.

Inclusion Criteria:

• Admission to PICU and intubated and ventilated (including via tracheostomy)
• Possible or proven lower respiratory tract infection

Exclusion Criteria:

• Imminent death or palliative care pathway planned

Children intubated in pediatric intensive care unit