Reliability of Kirby-Bauer Disk Diffusion Method for Detecting Doripenem Susceptibility in Oxidase Positive Non-Fermenting Gram Negative Bacilli

Doripenem is a member of the carbapenem family of antibiotics with significant potential for use in Pseudomonas aeruginosa infections without acquired resistance. United States Food and Drug Administration (US FDA) approved indications for doripenem include treatment of complicated urinary tract infections (cUTI), complicated intra-abdominal infections (cIAI) and nosocomial pneumonia including ventilator-associated pneumonia (VAP). Non-fermenting Gram negative bacilli (NFGNB) have emerged as important multi-drug resistant nosocomial pathogens and may be associated with poor clinical outcomes. The aim of this study therefore was to determine the susceptibility pattern of doripenem against clinical isolates of Pseudomonas spp and other NFGNB. In addition we also used the Kirby Bauer Disc Diffusion method to compare results of Doripenem susceptibility using discs (10μgm) procured from Hi-media and BD Diagnostics. A total of 80 clinical samples of patients with cUTIs and cIAIs were included in the study. Consecutive clinical isolates of oxidase positive NFGNB were identified by routine biochemical examination. Comparison of zone diameters of doripenem discs (10μgm) procured from BD diagnostics and Hi-Media was done with results of bioMérieux Etest. Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 were used as quality control strains. Susceptibility pattern of isolates was determined by both current CLSI guidelines as well as EUCAST (http://www.EUCAST.org). Discordant results were categorized as very major (sensitive according to KBDD method but resistant by E test), major (resistant according to KBDD method but sensitive by E test) and minor (reported intermediate when resistant or susceptible or vice versa).The isolates were identified as Pseudomonas aeruginosa (n=34), other Pseudomonas species (n=38) and Burkholderia cepacia complex (n=8). Very major errors and major errors were observed with two and one isolates respectively. Minor error rates of 7.5% were observed when results were interpreted as per CLSI break points. No discrepancies were recorded between KBDD and E test when EUCAST break-points were applied to the results obtained.