Determination of disc breakpoints and evaluation of Etests for tigecycline susceptibility testing by the BSAC method

Abstract

Tigecycline has been recently licensed in Europe for intra-abdominal and complicated skin and soft tissue infections. We determined zone breakpoints for use with 15 microg tigecycline discs and evaluated Etests for the routine determination of tigecycline susceptibility by BSAC methods. Disc zones for 2236 isolates and MICs by Etest for 531 isolates were compared with MICs obtained by the BSAC agar dilution method. Based on error minimization, we propose zone breakpoints for 15 microg tigecycline discs as follows: alpha/beta-haemolytic streptococci, S > or = 25 mm, R < or = 19 mm; Acinetobacter spp. and Enterobacteriaceae, S > or = 24 mm, R < or = 19 mm; Enterococcus spp., S > or = 21 mm, R < or = 20 mm; Haemophilus spp., S > or = 28 mm, R < or = 27; Streptococcus pneumoniae, S > or = 24 mm, R < or = 23 mm; and staphylococci, S > or = 26 mm, R < or = 25 mm. These criteria gave overall false resistance rates of < or =0.8% and false susceptibility rates of < or =0.7%. Tigecycline Etests, used on Iso-Sensitest agar, gave MICs within one doubling dilution of those by agar dilution in 97% of cases. Categorization agreement was good for isolates with borderline susceptibility or resistance-a group where Etests are likely to be used in order to verify disc-based results. MICs for highly susceptible alpha-haemolytic streptococci were underestimated by Etest, but this seems unlikely to be significant. Disc breakpoints corresponding to BSAC MIC breakpoints were defined for 15 microg tigecycline discs and have been adopted by the BSAC. Tigecycline Etest gave results in good agreement with agar dilution.