Severe infections can be caused by various organisms, such as bacteria or viruses, and lead to otherwise healthy people getting very unwell, sometimes needing treatment in hospital or even intensive care. For the treatment of bacterial infections to be successful, the correct antibiotics need to be given promptly. Early in the course of illness, clinicians often do not know exactly which bacteria are causing the infection. Furthermore, patients differ in terms of how their bodies process the antibiotics they are given; this means that some may get too much and others too little. This can in turn lead to some patients not being fully cured, and others coming to harm due to side effects of higher doses of these drugs.

For certain types of antibiotics, clinicians are able to measure their levels in the bloodstream, which can help guide dosing. This is called therapeutic drug monitoring, and is commonly used in clinical practice. One of the problems with therapeutic drug monitoring is that it is often not available outside of regular working hours, is costly, and most importantly, provides clinicians with useful information only after a few days of treatment have already been completed. This may be too late to treat these severely ill patients with life-threatening infections, where early and appropriate treatments matter.

The aim of our study, called TDM-TIME, is to look at how long it takes for blood samples to get from the patient to the laboratory to be measured, with the results then communicated back to clinicians. We are further looking to investigate whether steps can be taken to improve these timings, which would lead to shorter times until treatments can be improved. As our study is observational, we will not change anything about the treatment of our patients, but will only be measuring levels of antibiotics in their blood.

Inclusion Criteria:

• Age > 18 years;
• Admitted to intensive care;
• Treated for presumed or confirmed lower respiratory tract infection;
• Receiving OR about to receive the first dose of intravenous antimicrobials (either meropenem of piperacillin/tazobactam);
• Valid informed consent OR enrolment through deferred consent appropriate.

Exclusion Criteria:

• Severe anaemia (haemoglobin level < 70 g/L);
• Unlikely to survive 24 hours as judged by the treating physician;
• Study antimicrobial course started more than 24 hours ago.

All individuals will be considered for inclusion in this study regardless of age, disability, gender reassignment, marriage and civil partnership, pregnancy and maternity, race, religion and belief, sex, and sexual orientation except where the study inclusion and exclusion criteria explicitly state otherwise.