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A major reduction in hospital-onset Staphylococcus aureus bacteremia in Australia: a question of definition.

TO THE EDITOR—We read with interest the recent article reporting Staphylococcus aureus bacteremia (SAB) in Australia by Mitchell et al, noting a reduction in hospital-onset infections over a 12-year period [1]. Although their work is valuable in spanning a prolonged observation period and multiple Australian states, caution is required in using findings to estimate burden of illness. First, the SAB definition employed is not a nationally applied metric in Australia. For this study, hospital-onset SAB was defined as bacteremia occurring after 48 hours of hospitalization. This definition does not include early-onset SAB events (≤48 hours following hospitalization) that are directly associated with healthcare exposure. Patients managed in ambulatory or community settings, such as those with malignancy and those requiring hemodialysis, frequently have indwelling medical devices, which pose risk for SAB infection [2]. The definition of “healthcare-associated’’ appropriately includes these patients [3, 4], and is preferred to “nosocomial” or “hospital-onset” [5], which focus primarily on inpatient populations [6]. We have previously reported the burden of illness related to SAB in Australia using the standardized and more inclusive definition of healthcare-associated SAB [7], which includes early-onset infections (≤48 hours) when SAB was a complication of the presence of an indwelling medical device, SAB occurred within 30 days of a surgical procedure where the SAB was related to the surgical site, SAB was diagnosed within 48 hours of a related invasive instrumentation or incision, or SAB was associated with neutropenia contributed to by cytotoxic therapy [8]. Importantly, our data indicate that 26.2% of all healthcare-associated SAB events occur in the early period following hospitalization, and the majority of these were in association with an indwelling medical device [8]. These often preventable SAB events were not included in the study by Mitchell et al. We therefore suggest that the data provided are a significant underestimate of SAB infections, and that overall SAB cases associated with healthcare exposure would exceed 3700 for the studied 12-year period. Second, robust analysis of data over time is required, and we suggest that a model adjusting for heterogeneity at the level of the contributing hospitals (eg, by modeling hospital site as a random effect in a mixed model or incorporated into multistate frailty model) [9], with appropriate consideration of the linearity or nonlinearity of trend, provides the best fit for this type of data. Finally, the authors propose various explanations for a reduction in SAB events during the studied time frame, including national guidelines and hospital accreditation standards. We suggest that although these initiatives are important, many were not widely operational during the studied time frame (2002–2013). More likely, the observed SAB reduction during the earlier studied time frame is consistent with trends frequently seen in tandem with commencement of surveillance activities [10]. Appropriate management of indwelling medical devices is also an important contributing factor for SAB risk reduction, but relevant longitudinal data are not available in Australia for these practices to be aligned with study outcomes.