Multicenter studies of tigecycline disk diffusion susceptibility results for Acinetobacter spp

Abstract

Acinetobacter sp. isolates having multidrug resistance (MDR) patterns have become common in many medical centers worldwide, limiting therapeutic options. A five-center study tested 103 contemporary clinical Acinetobacter spp., including MDR strains, by reference broth microdilution and disk diffusion (15-mug disk content) methods against tigecycline. Applying U.S. Food and Drug Administration tigecycline breakpoint criteria for Enterobacteriaceae (susceptibility at < or =2 microg/ml [< or =1 microg/ml by the European Committee on Antimicrobial Susceptibility Testing]; disk diffusion breakpoints at > or =19 mm and < or =14 mm) to Acinetobacter spp. led to an unacceptable error rate (23.3%). However, an adjustment of tigecycline disk diffusion breakpoints (susceptible/resistant) to > or =16/ < or =12 mm reduced intermethod errors to an acceptable level (only 9.7%, all minor).

FIG. 1.

Scattergram comparing tigecycline MICs (μg/ml) and zones of inhibition around 15-μg tigecycline disks when tested against 103 Acinetobacter isolates. This was a multicenter (five-site) investigation with a diverse collection of recent clinical strains. The solid vertical and horizontal lines show the interpretive criteria for Enterobacteriaceae published in the U.S. FDA-approved product package insert (Tygacil package insert [June 2005], Wyeth Pharmaceuticals Inc., Philadelphia, PA) for CLSI methods (3, 4). The dashed vertical lines illustrate the proposed breakpoints for Acinetobacter sp. testing for two options (Table 1).