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Assessing the efficacy of HME filters at preventing contamination of breathing systems *

Breathing system filters are intended to prevent cross‐infection during anaesthesia. However, there is a lack of information on whether filters prevent contamination of the breathing system by the patient. We measured the contamination of 235 used filters of four different types obtained from operating theatres: two pleated hydrophobic (BB25M and BB22/15M, Pall Medical, Portsmouth, UK) used for adult patients and two electrostatic (355/5430 Hygroboy and 355/5427 Hygrobaby, Tyco Healthcare, Gosport, UK) used for paediatric patients. The filters were swabbed over their internal surfaces on both the patient and the machine sides and these were assessed with the use of adenosine triphosphate bioluminescence. Contamination was present on the machine side of 20 (9%) filters. Current standards for testing of filters has no set ‘pass’ level and is performed in the laboratory setting. Bioluminescence may be used in the clinical setting to elucidate factors that might increase the chance of cross‐contamination between patients.