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Background and Importance For antipseudomonal cephalosporins, the therapeutic goal is to maintain plasma levels at least four times above the minimal inhibitory concentration (MIC) of the target pathogen, while avoiding toxic concentrations (>35 µg/mL for cefepime and >80 µg/mL for ceftazidime). Standard dosing regimens of 6 g per day may result in toxic levels and subsequent adverse effects in certain patients depending on their clinical characteristics.Cefepime- and ceftazidime-induced neurotoxicity is an underdiagnosed adverse effect in critically ill patients, particularly in those under sedoanalgesia. In this context, therapeutic drug monitoring (TDM) enables the identification of supratherapeutic plasma concentrations that can guide therapy optimisation and prevention of neurotoxicity.Aim and Objectives The aim of this study is to describe the relevance of performing TDM of antipseudomonal cephalosporins to achieve pharmacokinetic/pharmacodynamic (PK/PD) targets whilst detecting drug-related neurotoxicity in critically ill patients, where neurological symptoms may remain unnoticed because of concomitant sedoanalgesia.Material and Methods This is a descriptive, single-centre study conducted over 9 months in critically ill patients in a third-level hospital.Baseline demographic and clinical data were collected, including age, sex, type of infection, causative microorganism, antibiotic choice, treatment regimen, plasma levels, renal function and neurotoxicity symptoms.Outcome included dose adjustments following TDM, and its correlation with neurotoxicity symptoms.Results 34 patients were included in the study, of which 82% (n=28) were male. The mean age was 64 (range 28-79). 94% (n=32) received continuous infusion and 68% (n=23) were empiric treatments. 44% (n =15) had supratherapeutic levels of which 66% (n=10) had developed neurotoxicity.100% (n=15) of patients with supratherapeutic levels had their dose reduced following TDM recommendation.Conclusion and Relevance Supratherapeutic plasma levels of cefepime and ceftazidime were significantly correlated with the occurrence of neurotoxicity in critically ill patients (p = 0.000023).TDM proved to be a valuable tool to identify patients at risk and to optimise antibiotic dosing.References and/or Acknowledgements 1. Roger C, et al. Beta-lactams toxicity in the intensive care unit: an underestimated collateral damage? Microorganisms 2021.2. Guilhaumou R, et al. Optimization of the treatment with beta-lactam antibiotics in critically ill patients-guidelines from the French society of pharmacology and therapeutics and the French society of anaesthesia and intensive care medicine. Critical Care 2019.Conflict of Interest No conflict of interest