Use of direct Etest in the management of ventilator-associated pneumonia due to resistant Gram-negative pathogens

Background

Increasing bacterial antibiotic resistance combined with the increasing incidence of Clostridium difficile disease complicate the choice of empirical therapy for ventilator-associated pneumonia (VAP). Etest strips rapidly adsorb a gradient of antimicrobial agent onto agar, allowing determination of minimum inhibitory concentration of cultured organisms. Their use on plates directly inoculated with respiratory samples to provide rapid susceptibility results in VAP has previously been described in a study in which the principal pathogens were Staphylococcus aureus and Pseudomonas aeruginosa. The current study evaluated the technique in a setting where resistant Enterobacteriaceae predominate.

Methods

Chromogenic Mueller–Hinton agar was inoculated with 100 respiratory specimens from patients clinically suspected to have VAP. Vancomycin, cefoxitin, cefotaxime, ceftazidime, piperacillin–tazobactam and meropenem Etest strips were then applied to the inoculated medium strips selected to aid detection of resistant Gram-negative pathogens. In addition, a P. aeruginosa diatab was applied to plates to facilitate identification of this organism. The plates were incubated at 37°C in 5% CO₂ overnight and subsequently interpreted using a prospectively designed protocol to suggest antimicrobial choice. Specimens were processed using UK standard methods in parallel to allow comparison of speed and accuracy.

Results

Forty-four out of 100 samples yielded no significant bacterial growth using the standard method. Sixty-three isolates were speciated from the remaining 56 samples (including 37 coliforms and 13 P. aeruginosa). Fifty-four of these samples had detectable growth at day 1 by the Etest method. Of the coliforms, 17 possessed extended-spectrum β-lactamase genes, including ampC, all detected by the Etest method. Organisms were confirmed susceptible to the Etest suggested antibiotic in 52/54 cases. Direct results would have led to a change in antibiotics to a more appropriate agent in 11 cases. An early indication of the presence of a multiresistant pathogen would have occurred in four cases. All Etest results were available within 24 hours, compared with a mean time to sensitivity reporting of 2.25 days using the standard method.

Conclusion

The direct Etest method provides rapid and accurate susceptibility results on patients with VAP, expediting selection of an antibiotic of appropriate spectrum.