Piperacillin-tazobactam use in ESBL Escherichia coli bacteremia: Should reporting be revised?

Background: While certain extended-spectrum beta-lactamase (ESBL) producing Enterobacteriaceae may appear susceptible to piperacillin-tazobactam in the laboratory, controversy exists regarding its appropriateness to treat serious infections. The Clinical and Laboratory Standards Institute (CLSI) recommends that piperacillin-tazobactam susceptibility be reported as tested, and that ESBL screening is not required in Enterobacteriaceae. We evaluated a sequential group of patients with ESBL Escherichia coli bacteremia to determine the factors associated with piperacillin-tazobactam use despite this controversy. Methods: This was a retrospective observational study at the McGill University Health Center (832 beds) in Montréal, Canada, from April 2010 to June 2015, examining patients with positive blood cultures that grew E. coli. Pediatric, untreated, and duplicate cultures within 14 days were excluded. Antimicrobial susceptibility testing was determined in accordance with CLSI guidelines. Definitive therapy was defined as antibiotics given after susceptibility results became available. Results: There were 845 E. coli bacteremias in the data set, with a median patient age of 68 years (IQR 57–81.25) and a 30-day in-hospital mortality of 12.1%. Overall, there were 44 ceftriaxone-resistant, piperacillin-tazobactam-susceptible bloodstream infections (BSI), and 41 received definitive therapy. Of these, 14/41 (34.1%) received piperacillin-tazobactam and 30 had an infectious disease consult performed. Consult with infectious disease was associated with less use of piperacillin-tazobactam as definitive therapy (7/30 [23%] versus 7/11 [64%]; OR 0.17; 95% CI 0.03–0.96; p = 0.02). Conclusion: Piperacillin-tazobactam definitive therapy was used in approximately one-third of ceftriaxone-resistant E. coli bacteremias. Given a risk of increased mortality with the use of beta-lactam/beta-lactamase inhibitors for treating serious ESBL infections, microbiology laboratories should consider withholding piperacillin-tazobactam susceptibility results in such cases pending definitive evidence of clinical efficacy.