Interpreting Susceptibility Testing of Carbapenem Against Pseudomonas aeruginosa

Objectives: Carbapenems are among the most powerful anti pseudomonal agents. Since meropenem and doripenem were marketed, there are limited data regarding drug susceptibility testing by routine methods (disc diffusion and Etest) for them. The aim of our study was to compare in vitro activity of the imipenem, meropenem and doripenem against Pseudomonas aeruginosa. Methods: Three hundred and eleven P. aeruginosa strains isolated from respiratory specimens in 170 patients who developed ventilator-associated pneumonia in two intensive care units were collected over a period of 31 months. The susceptibility of all of these isolates to imipenem, meropenem and doripenem were determined by Etest and disc diffusion method. Results: Considering either all of the isolates or only the first isolates recovered per patient (311 and 170 respectively) the susceptibility rate for doripenem was higher than that for meropenem and imipenem. When MICs determined by Etest were converted into interpretative categories (S, I, R) using French (CA-SFM) guidelines, agreement was poor, especially for meropenem and doripenem. The percent of agreement with the disc diffusion method were 90.6% and 89.7% for imipenem, 80.5% and 82.6% for meropenem and 80.5% and 73.3% for doripenem, for the first isolates and all of the isolates, respectively. Errors were mostly minor errors, and the rate of errors was as high as 17.7% and 16.1% for meropenem and 17.7% and 25.7% for doripenem for the first isolates and all of the isolates, respectively Conclusion: The accuracy of disc diffusion using CA-SFM guidelines appears unsatisfactory for all the three carbapenems justifying the adaptation of new guidelines for P. aeruginosa and carbapenems