• Often problematic in the critically ill patient as focal signs may be lacking and/or camouflaged by concurrent disease (e.g. ventilator-associated pneumonia on top of ARDS). Symptoms are often notforthcoming due to the patient's mentally incompetent state.

• In addition, all of the traditional clinical and biochemical markers of infection are non-specific. These include pyrexia, neutrophilia and altered sputum. Furthermore, the frequent presence of colonising organisms e.g. MRSA on skin, Pseudomonas aeruginosa in the respiratory tract, does not imply concomitant infection. As a consequence, many patients are over-treated with antibiotics, enhancing the risk ofovergrowth of resistant/atypical organisms.

• Markers of inflammation (C-reactive protein, procalcitonin) may be useful, though studies have produced conflicting results as to their specificity/sensitivity in diagnosing underlying infection.

• The value of routine screening (microbiological surveillance) is not proven, though this may help to identify infecting organisms earlier.

• For cases of suspected infection, appropriate samples should be taken for analysis including blood, sputum, wound swabs, drainage fluid, aspirated pus, catheter tips, cerebrospinal fluid, etc. These shouldgenerally be taken before new antibiotics are commenced.

Consider less common causes of infection such as endocarditis or osteomyelitis, particularly if the patient fails to settle after a standard course of therapy.

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