Abstract The substantial underlying disease burden, in combination with the therapeutic interventions provided, can result in significantly altered end-organ function in the critically ill. These changes can in turn affect key pharmacokinetic (PK) indices for many antibiotics, including drug clearance, promoting potentially subtherapeutic concentrations for lengthy periods of the dosing interval, therapeutic failure or the selection of resistant organisms. This paper presents three instructional cases from our tertiary-level Intensive Care Unit, where established antibiotic dosing regimens failed to achieve predefined PK targets for optimal bacterial killing. Using therapeutic drug monitoring (TDM), significant dose modification was subsequently undertaken. We propose augmented renal clearance as a possible mechanism underlying this phenomenon, particularly in young post-operative, burns or head-injured patients with normal serum creatinine concentrations. TDM, or at least a measured creatinine clearance, should be considered early in this setting to allow the optimisation of antibiotic exposure.