[Evaluation of disk diffusion test for carbapenem sensitivity in Acinetobacter baumannii and Pseudomonas aeruginosa...

This study was conducted upon the detection of discordant results obtained for Acinetobacter species against meropenem by disk diffusion susceptibility and E-test method. The aim of this study was to investigate the reliability of disk diffusion method for the determination of carbapenem susceptibility in non-fermentative bacilli. Carbapenem (imipenem and meropenem) sensitivites of 157 Acinetobacter baumanii and 157 Pseudomonas aeruginosa strains were performed by disk diffusion test and E-test in our laboratory where quality control of antibiotic susceptibility tests are performed weekly. For meropenem, discordance of the two tests were 10.8% and 4.5% for A. baumannii and P. aeruginosa, respectively. MIC values and disk diffusion zone diameters were significantly different from each other. There was no difference in terms of imipenem in A. baumannii strains but 5.1% of P. aeruginosa isolates showed discordance. Since, for the P. aeruginosa isolates with discordant imipenem susceptibility results, the imipenem disk diffusion zones were close to the lower level and their MIC values were within the intermediate resistance range, it was assumed that this technical discordance had low risk of reproducibility. In routine laboratories, meropenem resistance detected by disk diffusion method, should be verified by another technique. This is not only important for patient results but for epidemiological data as well. Routine quality control studies for antibiotic susceptibility tests and close observation of suspicious results, is mandatory for each laboratory, in order to achieve good quality performance.