BSAC standardized disc susceptibility testing method (version 10)

J Antimicrob Chemother 2011; 66: 2726 – 2757 doi:10.1093/jac/dkr359 Advance Access publication 15 September 2011 BSAC standardized disc susceptibility testing method (version 10) J. M. Andrews1* and R. A. Howe2 for the BSAC Working Party on Susceptibility Testing 1 Department of Microbiology, Sandwell and West Birmingham NHS Trust, City Hospital, Birmingham, B18 7QH, UK; 2NPHS Microbiology Cardiff, University Hospital of Wales, Heath Park, Cardiff, CF14 4XW, UK *Corresponding author. Tel: +44-121-507-5693; Fax: +44-121-507-5521; E-mail: The BSAC standardized disc susceptibility testing method remains unchanged, but there are considerable changes to the interpretative criteria due to continuing harmonization with the European Committee on Antimicrobial Susceptibility Testing (EUCAST) MIC breakpoints. There are a number of agents for which interpretative criteria have been removed. These MIC and/or zone diameter breakpoints will be published on the BSAC web site as a ‘Legacy’ table; they may be used for research or comparative purposes, but are not recommended for clinical management. Notably, testing of staphylococci for susceptibility to glycopeptides by disc diffusion has been removed because this method has been found to be unreliable, particularly for the detection of low-level resistance; low-level vancomycin resistance in staphylococci is increasingly deemed to be of clinical relevance. The tables for anaerobes have been expanded to include MIC breakpoints that have been determined by EUCAST. There are currently no zone diameter breakpoints for these organisms and an MIC method is recommended if susceptibility testing is required. Keywords: breakpoints, disc testing, MICs Introduction Version 10 of the BSAC standardized disc susceptibility testing method has been published on the BSAC web site (http://www. bsac.org.uk/Susceptibility+Testing/GUIDELINES+Standardized+ Disc+Susceptibility+Testing+Method). The basic disc susceptibility testing method remains unchanged,1 but there have been a number of alterations to the interpretive criteria due to continuing harmonization with the European Committee on Antimicrobial Susceptibility Testing (EUCAST)2 MIC breakpoints, and constant efforts to improve the reliability and clinical applicability of the guidance. Instances where changes have been made to the interpretative criteria for aerobic bacteria tested with different antibiotics are shown in Table 1. The change introduced may have been to the MIC breakpoint, the zone diameter interpretation and/or the comment column of the Table, while in other instances (e.g. piperacillin/tazobactam for Pseudomonas spp.) a review of the data has led to alteration of the zone criteria although the MIC breakpoint remains the same. However, there are also instances (e.g. cefepime for Enterobacteriaceae) where the MIC breakpoint has been altered but no change in zone diameter criteria is required. This is because zone diameter criteria are determined using population distributions of zone diameters for organisms with different MICs and resistance mechanisms, to select the most effective and reproducible breakpoint to differentiate organisms within different MIC susceptibility categories rather than simple regression. In certain cases, the BSAC has maintained a breakpoint where one has not been determined by EUCAST; this is usually where there has been no requirement for a Europe-wide breakpoint for a compound due to limited availability of the compound in different countries (e.g. temocillin). Table 2 shows the changes to the interpretive criteria for anaerobic bacteria. A number of new MIC breakpoints that have been determined by EUCAST have been added. There are currently no zone diameter criteria for these agents and an MIC method is recommended if susceptibility testing is required. For the few compounds where disc criteria exist, it is highlighted that these only apply to Bacteroides fragilis, Bacteroides thetaiotaomicron or Clostridium perfringens, as indicated. There are a number of compounds, listed in Table 3, for which interpretive criteria have been removed. This is either because the species is considered a poor target for therapy with the drug (e.g. Pseudomonas spp. and moxifloxacin), susceptibility can be most effectively inferred from susceptibility to other agents (e.g. ertapenem susceptibility of b-haemolytic streptococci can be inferred from the penicillin result), or the agent is no longer widely available or used (e.g. carbenicillin). These MIC breakpoints and zone diameter criteria will be published separately on the BSAC web site as a ‘Legacy’ table. They may be used for research or comparative purposes, but it is not recommended to use the legacy data for clinical management as their applicability is uncertain and the breakpoints will no longer be maintained. # The Author 2011. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: 2726 JAC BSAC standardized susceptibility testing method (version 10) Table 1. Continued Table 1. Changes to interpretive tables for aerobic bacteria Organism/antibiotic Enterobacteriaceae (Table 4) Aminoglycosides Amoxicillin/ampicillin Co-amoxiclav Mecillinam Piperacillin Piperacillin/tazobactam Temocillin (systemic and UTI) Ticarcillin/clavulanate Cefuroxime Ceftriaxone Cefalexin Cefixime Cefotaxime Cefepime Cefoxitin (AmpC screen) Cefpodoxime (ESBL screen) Ceftazidime Doripenem Ertapenem Imipenem Meropenem Aztreonam Norfloxacin UTI Levofloxacin Moxifloxacin Nalidixic acid Ofloxacin Ciprofloxacin Norfloxacin (systemic) Norfloxacin UTI Azithromycin Tigecycline Chloramphenicol Co-trimoxazole Trimethoprim Fosfomycin Nitrofurantoin Colistin Acinetobacter spp. (Table 5) Gentamicin Amikacin Doripenem Imipenem Meropenem Ciprofloxacin Tigecycline Piperacillin/tazobactam Colistin MIC Breakpoint Zone diameter interpretation N N N Y N Y Y Comment N Y Y N Y Y N N N N N Y N N N N N Y Y Y Y Y N N N N N N N N N N Y Y N N N N N N N N N N N N N N Y N Y Y N N N N N Y Y Y N Y Y Y Y Y Y Y Y Y Y Y N N N N N Y Y Y N N N N N N N Y N Y Y Y Continued Organism/antibiotic Pseudomonas spp. (Table 6) Aminoglycosides Ticarcillin Ticarcillin/clavulanate Ceftazidime Aztreonam Quinolones Piperacillin Piperacillin/tazobactam Doripenem Imipenem Meropenem Colistin Staphylococci (Table 8) Aminoglycosides Oxacillin Ciprofloxacin (systemic) Moxifloxacin Ofloxacin Azithromycin Clarithromycin Clindamycin Quinupristin/dalfopristin Doxycycline Minocycline Daptomycin Chloramphenicol Co-trimoxazole Trimethoprim UTI Fosfomycin (intravenous) Fusidic acid Linezolid Mupirocin Nitrofurantoin UTI Rifampicin b-Lactams Ampicillin (UTI) Penicillin Cefoxitin Ciprofloxacin (UTI) Teicoplanin Vancomycin Erythromycin Tetracycline Tigecycline Trimethoprim MIC Breakpoint Zone diameter interpretation N N N N N N N N N N N N Y Y Y Y N N N N N N N N N N (...truncated)