title: Control-group selection importance in studies of antimicrobial resistance: examples applied to Pseudomonas aeruginosa, Enterococci, and Escherichia coli.
abstract: We aimed to illustrate the importance of control-group selection on the results of risk factor analysis for (1) imipenem-resistant Pseudomonas aeruginosa, (2) vancomycin-resistant enterococci (VRE), and (3) ampicillin-sulbactam-resistant Escherichia coli. Case patients were compared with 2 different control groups: patients with the susceptible form of the organism (type 1), and control patients among whom the case patients arose during the same period as the case patients (type 2). Comparison of case patients who had imipenem-resistant P. aeruginosa with type-1 control patients identified use of imipenem (odds ratio [OR], 27.1) and quinolones (OR, 3.25) as a risk factor for selection of antimicrobial resistance, and comparison of the same case patients with type-2 control patients identified imipenem (OR, 6.34). When case patients with VRE were compared with type-1 and with type-2 control patients, use of vancomycin was identified as a risk factor (OR, 4.38 and 2.77, respectively). Comparison of case patients who had ampicillin-sulbactam-resistant E. coli compared with type-1 control patients identified ampicillin-sulbactam (OR, 2.71) and quinolones (OR, 2.72), and comparison with type-2 control patients identified ampicillin-sulbactam (OR, 1.68). The selection of control patients from the potentially suboptimal control type 1 can falsely identify certain antibiotics and overestimate the OR of the resistance-defining antibiotic.
authors.affiliation: Department of Epidemiology and Preventive Medicine, University of Maryland School of Medicine, and VA Maryland Health Care System, Baltimore, MD, USA. aharris@epi.umaryland.edu
authors.forename: Anthony D
authors.initials: AD
authors.lastname: Harris
authors.forename: Matthew H
authors.initials: MH
authors.lastname: Samore
authors.forename: Marc
authors.initials: M
authors.lastname: Lipsitch
authors.forename: Keith S
authors.initials: KS
authors.lastname: Kaye
authors.forename: Eli
authors.initials: E
authors.lastname: Perencevich
authors.forename: Yehuda
authors.initials: Y
authors.lastname: Carmeli
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D016022:Case-Control Studies; D024881:Drug Resistance, Bacterial / Q000235:genetics; D016983:Enterococcus / Q000187:drug effects*; D004926:Escherichia coli / Q000187:drug effects*; D005787:Gene Frequency; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods; D015999:Multivariate Analysis; D016017:Odds Ratio; D011550:Pseudomonas aeruginosa / Q000187:drug effects*; D012107:Research Design; D012307:Risk Factors
chemical_list: D000900:Anti-Bacterial Agents