Risk factors for imipenem-resistant Pseudomonas aeruginosa: a comparative analysis of two case–control studies in hospitalized patients

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Summary

Risk factors for acquisition of imipenem-resistant Pseudomonas aeruginosa by hospitalized patients were assessed at a tertiary care hospital. Two case–control studies with different control groups were used. In Study 1, patients with imipenem-resistant P. aeruginosa (IRPA) (case group) were compared with patients selected at random from the same unit. In Study 2, the case group was compared with patients with imipenem-susceptible P. aeruginosa (ISPA). Ninety-three patients with IRPA and 93 control patients were included in Study 1, and 93 IRPA patients and 65 patients with ISPA were included in Study 2. Carbapenem treatment [odds ratio (OR) 5.82], mechanical ventilation (OR 3.22) and hospital admission in the previous year (OR 2.59) were associated with IRPA in Study 1. An interaction between carbapenem and vancomycin was found to be a significant risk factor for IRPA (OR for carbapenem in patients with vancomycin use 43.71). In Study 2, carbapenem exposure (OR 12.82) and renal failure (OR 5.00) were associated with IRPA. Our study confirmed that carbapenem exposure is the main risk factor for IRPA, and found that the use of both carbapenem and vancomycin can increase this effect.

Introduction

Pseudomonas aeruginosa is a leading cause of nosocomial infection, particularly pneumonia, according to recent surveillance reports.1, 2 This organism is also involved in urinary, wound and bloodstream infections. Infections due to P. aeruginosa are difficult to treat because of the limited therapeutic options, and are usually associated with high mortality despite the use of appropriate therapy.3, 4 The resistance of P. aeruginosa to imipenem has been reported worldwide5, 6, 7 and is often associated with resistance to other antipseudomonal drugs.8, 9

Inadequate empirical antimicrobial therapy is associated with adverse outcomes,10, 11, 12, 13 and the identification of risk factors for antimicrobial resistance in P. aeruginosa could guide clinicians in their empirical therapeutic options for individual patients. The identification of risk factors may also lead to changes in antibiotic-prescribing patterns, with a decrease in bacterial resistance and improvement of outcome for these patients.14

Previous studies have examined risk factors for antimicrobial resistance with emphasis on the control group selection.14, 15, 16 The aim of the present study was to identify clinically significant risk factors for acquiring imipenem-resistant P. aeruginosa, and to assess risk factors reported previously using a comparative approach of two case–control studies with distinct control groups.

Section snippets

Methods

Two case–control studies were performed concurrently at Hospital São Lucas, a 600-bed tertiary care teaching hospital. All patients who had nosocomial P. aeruginosa isolated from any site between 1 September 2002 and 1 July 2003 were included in the study. In Study 1, case patients were those with imipenem-resistant P. aeruginosa (IRPA) and control patients were selected at random from patients hospitalized in the same unit by means of computer-generated random numbers. In Study 2, cases were

Results

In total, 189 P. aeruginosa isolates were identified by the microbiology laboratory. Ten were excluded because they were isolated from patients within 48 h of hospital admission, and 16 because they were isolated from patients with previous IRPA (N=4) or ISPA (N=12). Five patients who were initially selected as controls in Study 2 yielded IRPA and were transferred to the case groups. Two patients initially selected as controls in Study 1 had IRPA and ISPA and were transferred to the case group

Discussion

The present study assessed potential risk factors for IRPA in hospital patients. We performed two studies with distinct control groups for better understanding of the association of the variables with the outcome.

In studying risk factors for the isolation of nosocomial-resistant bacteria, control patients selected from the same unit as the case patients are considered to be the most suitable.14, 15, 16, 20 However, this design cannot differentiate whether the variable analysed is a risk factor

Acknowledgements

We thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Ministry of Education of Brazil for financial support; Dr Luis Fernando Rodrigues, Cláudia Meirelles Leite and Fabiana Correa Soares for microbiology laboratory support; Dr Mario B. Wagner for statistical analysis; and Andrea Escobar Dias for database management.

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