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

Background: 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 mu g tigecycline discs and evaluated Etests for the routine determination of tigecycline susceptibility by BSAC methods. Methods: Disc zones for 2236 isolates and MICs by Etest for 531 isolates were compared with MICs obtained by the BSAC agar dilution method. Results: Based on error minimization, we propose zone breakpoints for 15 mg tigecycline discs as follows: alpha/beta-haemolytic streptococci, S >= 25 mm, R <= 19 mm; Acinetobacter spp. and Enterobacteriaceae, S >= 24 mm, R <= 19 mm; Enterococcus spp., S >= 21 mm, R <= 20 mm; Haemophilus spp., S >= 28 mm, R <= 27; Streptococcus pneumoniae, S >= 24 mm, R <= 23 mm; and staphylococci, S >= 26 mm, R <= 25 mm. These criteria gave overall false resistance rates of <= 0.8% and false susceptibility rates of <= 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. Conclusions: Disc breakpoints corresponding to BSAC MIC breakpoints were defined for 15 mu g tigecycline discs and have been adopted by the BSAC. Tigecycline Etest gave results in good agreement with agar dilution.