title: Comparison of commonly used antimicrobial susceptibility testing methods for evaluating susceptibilities of clinical isolates of Enterobacteriaceae and nonfermentative Gram-negative bacilli to cefoperazone-sulbactam.
abstract: OBJECTIVE
  The aim of this study was to investigate the cefoperazone-sulbactam (CFP-SUL) susceptibilities of important Gram-negative bacteria (GNB) by agar dilution (reference method), disk diffusion, and two automated methods.
  
  
  METHODS
  A total of 799 GNB isolates, including Enterobacteriaceae (n = 500) and nonfermentative GNB (NFGNB, n = 299), were recovered from various clinical specimens collected at National Taiwan University Hospital, Taipei, Taiwan from November 2013 to December 2014. The agar dilution method, disk diffusion method, and two automated susceptibility systems (Phoenix and Vitek 2) were used for testing susceptibility of the isolates to CFP-SUL. Categories of susceptibility (susceptible, intermediate, or resistant) to CFP-SUL yielded from each method were interpreted according to CFP-SUL interpretive breakpoints proposed previously. The results of categorical agreement and errors obtained between the agar dilution method and the other three methods were analyzed.
  
  
  RESULTS
  The Vitek 2 system had the highest error rates against Escherichia coli (n = 150) and Enterobacter cloacae (n = 77) isolates, i.e., 6.7% and 11.7% minor errors, 8.5% and 1.7% major errors, and 40% and 20% very major errors, respectively. Additionally, the Vitek 2 system was also found to have a significantly lower sensitivity (44.4%) and lower positive predictive value (18.2%) for detecting CFP-SUL nonsusceptible E. coli isolates than other methods. For carbapenem-nonsusceptible Enterobacteriaceae isolates, the Vitek 2 system failed to detect correct susceptibility to CFP-SUL. The three methods failed to correctly detect CFP-SUL susceptibility categories against all NFGNB isolates except Pseudomonas aeruginosa.
  
  
  CONCLUSIONS
  The Vitek 2 system is a suboptimal method in correctly detecting CFP-SUL susceptibility categories for E. coli, E. cloacae, and carbapenem-nonsusceptible Enterobacteriaceae isolates.
authors.affiliation: Emergency Department, Department of Emergency and Critical Care Medicine, Wan Fang Hospital, Taipei Medical University, Taipei, Taiwan.
authors.forename: Shio-Shin
authors.initials: SS
authors.lastname: Jean
authors.affiliation: Department of Internal Medicine, Far Eastern Memorial Hospital, New Taipei City, Taiwan.
authors.forename: Chun-Hsing
authors.initials: CH
authors.lastname: Liao
authors.affiliation: Department of Internal Medicine, National Taiwan University Hospital, National Taiwan University College of Medicine, Taipei, Taiwan.
authors.forename: Wang-Huei
authors.initials: WH
authors.lastname: Sheng
authors.affiliation: Division of Infectious Diseases, Department of Internal Medicine, Wan Fang Hospital, Taipei Medical University, Taipei, Taiwan.
authors.forename: Wen-Sen
authors.initials: WS
authors.lastname: Lee
authors.affiliation: Department of Internal Medicine, National Taiwan University Hospital, National Taiwan University College of Medicine, Taipei, Taiwan; Department of Laboratory Medicine, National Taiwan University Hospital, National Taiwan University College of Medicine, Taipei, Taiwan. Electronic address: hsporen@ntu.edu.tw.
authors.forename: Po-Ren
authors.initials: PR
authors.lastname: Hsueh
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D057205:Automation, Laboratory / Q000379:methods; D002438:Cefoperazone / Q000494:pharmacology*; D006090:Gram-Negative Bacteria / Q000187:drug effects* / Q000302:isolation & purification; D016905:Gram-Negative Bacterial Infections / Q000382:microbiology*; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods*; D013407:Sulbactam / Q000494:pharmacology*; D013624:Taiwan; D065093:beta-Lactamase Inhibitors / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D065093:beta-Lactamase Inhibitors; D002438:Cefoperazone; D013407:Sulbactam
keywords: Vitek 2 system; cefoperazone–sulbactam; disk diffusion; phoenix system; susceptibility test