Comparison of direct antimicrobial susceptibility testing methods for rapid analysis of bronchial secretion samples in ventilator-associated pneumonia

4th Department of Internal Medicine, Infectious Diseases Laboratory, University of Athens Medical School, Athens, Greece.
International journal of antimicrobial agents (Impact Factor: 4.26). 08/2011; 38(2):130-4. DOI: 10.1016/j.ijantimicag.2011.04.011
Source: PubMed

ABSTRACT Two hundred and fifty tracheal aspirates were subjected to direct antimicrobial susceptibility testing by disk diffusion, Etest and inoculation on antibiotic-enriched MacConkey agar plates. Results were compared with those obtained using an automated system on microorganisms recovered from standard quantitative culture. A total of 255 microorganisms were isolated from 194 positive samples by the standard quantitative procedure. A total of 85.1%, 82.5% and 72.5% agreement between direct disk diffusion, Etest and antibiotic-enriched MacConkey agar plates, respectively, and the standard procedure was observed in 64 microorganisms obtained from monomicrobial cultures that corresponded to 240 individual microorganism-antimicrobial agent combinations. Three (1.3%) and four (1.7%) very major errors for direct disk diffusion and Etest methods were observed, respectively. The antibiotic-enriched MacConkey agar plate method compared with the standard procedure demonstrated an unacceptable rate of very major (6.7%) and major errors (14.2%). Clinical evaluation of direct susceptibility tests based on the speculative impact on clinical practice by guiding patient's early treatment, if all positive cultures corresponded to infection, was correct in 79.9% for the direct disk diffusion test, 77.8% for the direct Etest method and 68.0% for antibiotic-enriched MacConkey agar plates. Direct diffusion tests (Etest or disk diffusion) applied on respiratory samples are rapid techniques that provide results comparable with standard antimicrobial susceptibility testing in <24 h.

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    ABSTRACT: Inappropriate antibiotic therapy in ventilator-associated pneumonia (VAP) is associated with increased mortality. Using broad-spectrum antibiotics for 48 h until the results of conventional cultures and antimicrobial susceptibility testing (AST) are available, may promote the emergence of drug-resistant bacteria. Performing AST directly on clinical respiratory samples would hasten the process by at least 24 h. Here, we analysed the diagnostic performance of a rapid method combining mass spectrometry and direct AST (DAST), and compared it with the conventional method (mass spectrometry with conventional AST (CAST)). Additionally, we assessed its potential impact on antimicrobial use in patients. Over a period of 18 months, the two methods were performed on 85 bronchoalveolar lavages obtained from intensive care unit patients with suspected VAP, and in which Gram-negative bacilli were observed on direct examination. Only the CAST results were reported to the clinicians. DAST produced useable results in 85.9% of the patients. The sensitivity and negative predictive values of DAST were 100% for all antibiotics tested, except gentamicin (97.1%, (95% CI 93.3-101) and 97.4% (93.7-101), respectively) and amikacin (88.9% (81.7-96.1) and 96.4% (92.1-100.7), respectively), compared with CAST. Specificity and positive predictive values ranged from 82.9 (74.2-91.5) to 100%, and from 86.4 (78.5-94.2) to 100%, respectively. If the DAST results had been reported to the clinicians, treatment could have been optimized 24 h earlier in 35/85 (41.2%) patients, with 17 carbapenem patient-days saved. Overall, routine use of the DAST method could help optimize earlier antibiotic treatment in patients with suspected VAP. Copyright © 2014 European Society of Clinical Microbiology and Infectious Diseases. Published by Elsevier Ltd. All rights reserved.
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