BSAC standardized disc susceptibility testing method (version 8)

Journal of Antimicrobial Chemotherapy (2009) 64, 454– 489 doi:10.1093/jac/dkp244 Advance Access publication 8 July 2009 BSAC standardized disc susceptibility testing method (version 8) J. M. Andrews* for the BSAC Working Party on Susceptibility Testing Department of Microbiology, Sandwell and West Birmingham NHS Trust, City Hospital, Birmingham, B18 7QH, UK There have been considerable changes to the format of the recommendations since the previous version (version 7). The majority of the footnotes to the tables have been removed and the notations added to the end column; it is hoped that this change will avoid confusion in interpretation. Antibiotics have been separated into groups, e.g. b-lactams, aminoglycosides, etc. Recommendations for urinary tract infections (UTIs) have been removed for most agents except for those that are administered solely for the treatment of uncomplicated UTIs or where there are limited recommendations for specific organisms, e.g. trimethoprim. For agents that previously had dual recommendations, systemic recommendations remain and the intermediate category can be used for interpretation for UTIs because intermediate susceptibility infers that the infection may respond as the agent is concentrated at the site of infection. This change will also avoid errors in interpretation when an organism is isolated from multiple sites, e.g. blood and urine. The changes that have been made to version 7 are as follows: MIC and zone diameter breakpoints (BPs) for trimethoprim, fosfomycin and nitrofurantoin for UTIs (Table 7); MIC and zone diameter breakpoints (BPs) for doripenem (Tables 7– 9); colistin MIC BPs for Pseudomonas spp. (Table 9), co-trimoxazole MIC BPs for Stenotrophomonas maltophilia (Table 10); staphylococci MIC and zone diameter BPs for clarithromycin, clindamycin, erythromycin, quinupristin/ dalfopristin, trimethoprim UTI, nitrofurantoin UTI and rifampicin (Table 11); Streptococcus pneumoniae MIC and zone diameter BPs for azithromycin, clarithromycin, erythromycin, co-trimoxazole, linezolid, rifampicin and telithromycin (Table 12); addition of streptomycin recommendations for enterococci (Table 13); enterococcal MIC and zone diameter BPs for quinupristin/dalfopristin, nitrofurantoin UTI and trimethoprim UTI (Table 13); b-haemolytic streptococci MIC and zone diameter BPs for azithromycin, clarithromycin, erythromycin and telithromycin (Table 15); clarithromycin and erythromycin MIC and zone diameter BPs for Moraxella catarrhalis (Table 16); azithromycin MIC BPs for Neisseria gonorrhoeae (Table 17); chloramphenicol and rifampicin MIC BPs for Neisseria meningitidis (Table 18); azithromycin MIC BPs for Haemophilus influenzae (Table 19); MIC BPs for metronidazole for Bacteroides fragilis, Bacteroides thetaiotaomicron and Clostridium perfringens (Tables 23–25, respectively); susceptibility testing of Listeria spp. (Appendix 3); the acceptable range for ATCC 25923 to a 10 mg tobramycin disc (Table 26). Keywords: breakpoints, disc testing, MICs Introduction The BSAC Guide to Sensitivity Testing was first published in 1991 and one of its most important sections was that dealing with breakpoints for clinically relevant bacteria.1 These breakpoints have been used extensively to interpret MIC results and for single concentration ‘breakpoint’ tests. However, a criticism of the guidelines was that they did not provide a standardized method of disc diffusion testing with zone limits that correlated with these MIC breakpoints. The limitations of the widely used Stokes’ comparative method were also a cause for concern. The task of developing such a method of disc testing is immense and the Working Party and the Council of the BSAC needed evidence that there was sufficient interest to warrant the investment required not only in the short term, but also for continuing support and development. This necessary confirmation was obtained from a questionnaire survey,2 which indicated that 90.6% of UK laboratories would be prepared to switch to an ..................................................................................................................................................................................................................................................................................................................................................................................................................................... *Corresponding author. Tel: þ44-121-507-5693; Fax: þ44-121-507-5521; E-mail: ..................................................................................................................................................................................................................................................................................................................................................................................................................................... 454 # The Author 2009. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: Received 5 June 2009; returned 11 June 2009; revised 12 June 2009; accepted 18 June 2009 BSAC standardized susceptibility testing method (version 8) 1 Preparation of plates 1.1 Prepare Iso-Sensitest agar (ISA; Oxoid, Basingstoke, UK), or media shown to have the same performance as ISA, according to the manufacturer’s instructions. Supplement media for fastidious organisms with 5% defibrinated horse blood or 5% defibrinated horse blood and 20 mg/L b-nicotinamide adenine dinucleotide (NAD) as indicated in Table 1. Use Columbia agar with 2% NaCl for methicillin/oxacillin susceptibility testing of staphylococci. 1.2 Pour sufficient molten agar into sterile Petri dishes to give a depth of 4+0.5 mm (25 mL in a 90 mm Petri dish). 1.3 Dry the surface of the agar to remove excess moisture before use. The length of time needed to dry the surface of the agar depends on the drying conditions, e.g. whether a fan-assisted drying cabinet or ‘still air’ incubator is used, whether plates are dried before storage and storage conditions. It is important that plates are not over-dried. 1.4 Store the plates in vented plastic boxes at 8 –108C prior to use. Alternatively the plates may be stored at 4 – 88C in sealed plastic bags. Plate drying, method of storage and storage time should be determined by individual laboratories as part of their quality assurance programme. In particular, quality control tests should confirm that excess surface moisture is not produced and that plates are not over-dried. 2 Selection of control organisms 2.1 The performance of the tests should be monitored by the use of appropriate control strains. The control strains listed (Tables 2 and 3) include susceptible strains that have been chosen to monitor test performance and resistant strains that can be used to confirm that the method will detect phenotypically resistant (...truncated)