792

Abstract

Introduction: Pseudomonas aeruginosa (PA) represents an important cause of bacteremia in critically ill patients and can result in substantial mortality. Initially appropriate antibiotic therapy (IAAT) is a key determinant of outcome in bacteremia. One component of IAAT reflects the in vitro activity of the antibiotic against the culprit pathogen. Administration of an agent to which the organism is not susceptible(NonSus) per se defines inappropriate therapy. Recently, the break points defining NonSus have changed for PA relative to piperacillin/tazobactam(PT). It is unclear how this alteration will change potential rates of IAAT for PA bacteremia. Hypothesis: We hypothesized that the new rubric defining NonSus for PA to PT would significantly decrease the rate of IAAT with empiric PT use. Methods: We retrospectively identified all cases of PA bacteremia at our institution between Jan 2010 and May 2012. We only included first episodes of bacteremia and excluded all duplicate isolates. The minimum inhibitory concentration 90 (MIC90) of the PA to PT was determined via disk diffusion by trained lab personnel. MIC90s were defined as NonSus based on both the older and now more recent recommendations of the Clinical Laboratory Standard Institute (CLSI). Under the older system, organisms with an MIC90 of >64/4 mg/dl were deemed NonSus while with the newer rubric isolates with an MIC90 of >16/4 mg/dl are categorized as NonSus. Rates of NonSus served as our primary endpoint. We conducted a subgroup analysis in those isolates that were recovered from critically ill patients. Results: The cohort included 98 cases of bacteremia. The median MIC90 to PT was 8/4 mg/dl. Under the older CLSI scheme, 13.3% of isolates were NonSus to PT compared to 20.4% with the new criteria. This approximate 7% absolute increase in rates of NonSus and thus potentially inappropriate therapy was not significant(p=0.252). Among critically ill subjects(n=49) rates of NonSus would increase from 16.3% to 19.7% (p=0.805). Conclusions: The newer CLSI approach to defining the breakpoints for PA to PT does not alter the proportion of patients classified as having NonSus isolates, irrespective of patient severity of illness. This suggests that rates of IAAT with empiric PT for PA bacteremia are unlikely to be altered purely because of the recent changes proposed by CLSI.