Evaluation of Etests for determining tigecycline MICs by BSAC methodology

Abstract number: 1733_1143

Hope R., Mushtaq S., Livermore D.

Introduction: Etests are calibrated against MICs determined using Clinical and Laboratory Standards Institute (CLSI) methods, with Mueller Hinton media. Most UK laboratories however use British Society for Antimicrobial Chemotherapy (BSAC) susceptibility testing methods, with IsoSensitest agar. We tested the agreement between tigecycline MICs determined by Etest and BSAC agar dilution methods.

Method: Isolates (n = 558) were selected from the BSAC 2004 bacteraemia surveillance collection and other collections held at the reference laboratory. They comprised: 63 Acinetobacter spp., 44 alpha-haemolytic streptococci, 45 beta-haemolytic streptococci, 35 coagulase-negative staphylococci, 55 Enterobacter, 40 enterococci, 35 E. coli, 46 Haemophilus, 54 Klebsiella, 55 Proteeae, 21 Serratia, 35 S. pneumoniae, and 30 S. aureus, selected to cover the broadest range of tigecycline MICs and to span the EUCAST/BSAC S/I/R breakpoints. Tigecycline MICs were determined in parallel using Etests, with confluent growth on IsoSensitest agar, and by the BSAC agar dilution method.

Results: MICs by agar dilution and Etest were within experimental error (i.e. one doubling dilution) in 527/558 (94%) of cases and within two dilutions in all cases. MICs by Etest nevertheless tended to be slightly lower than by agar dilution (lower in 33% of cases vs. higher in 10.5%), with this effect most evident for S. pneumoniae and other alpha-haemolytic streptococci, all of which were very susceptible, (MIC ≤ 0.5 mg/L). alpha-haemolytic streptococci accounted for 22/27 cases where MICs by Etest were 2 tubes lower than by agar dilution, despite using a hand lens to seek microcolonies and the final termination of growth. Among Gram-negative isolates (n = 113) with agar dilution MICs in the range 1–4 mg/L (i.e. spanning the ``top'' end of susceptible, through intermediate, to the ``bottom'' end of resistant with respect to EUCAST/BSAC breakpoints), catergorisation agreement between agar dilution and Etest MICs was 86%, with 14% minor errors and no major or very major errors.

Conclusion: Tigecycline Etests used on IsoSensitest agar gave good agreement with MICs determined using BSAC methodology. Categorisation agreement was good for isolates with borderline susceptibility or resistance – a group where Etests are likely to be used to verify disc based results. MICs for highly-susceptible alpha-haemolytic streptococci tended to be underestimated by Etest, but this seems unlikely to be consequential.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
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