JAC Advance Access originally published online on March 31, 2004
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Journal of Antimicrobial Chemotherapy (2004) 53, 729-738
© 2004 The British Society for Antimicrobial Chemotherapy
Reliability of routine disc susceptibility testing by the British Society for Antimicrobial Chemotherapy (BSAC) method
Antibiotic Resistance Monitoring & Reference Laboratory, Health Protection Agency, Specialist & Reference Microbiology Division, 61 Colindale Avenue, London NW9 5HT, UK
Received 19 December 2003; returned 12 January 2004; revised 27 February 2004; accepted 27 February 2004
Objectives: To ascertain the agreement between MICs determined at a central laboratory, and susceptible, intermediate and resistant categorizations based on zone diameters recorded at diagnostic laboratories using the BSAC standardized method.
Methods: Standardized disc susceptibility tests were performed at sentinel laboratories in three surveys, with MIC tests performed on the collected isolates at a reference laboratory. The organisms comprised over 3300 Enterobacteriaceae, Acinetobacter spp., pseudomonads, staphylococci and enterococci, with over 29 000 antibiotic/organism tests in total.
Results: More than 90% of the antibiotic/organism combinations classed as susceptible by disc tests in the sentinel laboratories were confirmed by MIC testing. Disagreements were more frequent where disc tests indicated resistance, with half of the piperacillin/tazobactam resistance and one-third of the cephalosporin resistance found in Enterobacteriaceae by disc tests not being confirmed, and with three-quarters of teicoplanin resistance in enterococci not confirmed. None of the few apparent cases of meropenem resistance in Enterobacteriaceae or linezolid, quinupristin/dalfopristin or vancomycin resistance in staphylococci were confirmed by MIC testing. When disagreements were found between disc- and MIC-based categorization, MICs were commonly, although not invariably, one to three doubling dilutions above or below the breakpoint. However, many of the disagreements where MICs were three or more dilutions from the breakpoint were not seen when disc tests were repeated in the central laboratory.
Conclusions: The BSAC disc method seems adequate for confirming susceptibility to guide therapy and to monitor resistance trends. Nevertheless, there must be concern about the over-estimation of many resistances, and frequent zone:MIC disagreements for isolates with borderline susceptibility.
Keywords: minimum inhibitory concentration, zone diameter, standardized method
* Corresponding author. Tel: +44-20-8200-6868; Fax: +44-20-8205-9185; E-mail Nicola.Potz{at}hpa.org.uk.
Present address. Healthcare-Associated Infection and Antimicrobial Resistance Department, HPA Communicable Disease Surveillance Centre, Colindale, London NW9 5EQ, UK.
¶ Present address. Wells Healthcare Communications Ltd., Speldhurst Place, Speldhurst Road, Tunbridge Wells, Kent, TN4 0JB, UK
Present address. St. George’s Healthcare NHS Trust, Blackshaw Road, London SW17 0QT, UK.
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