JAC Advance Access published online on March 31, 2004
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkh212
© 2004 by The British Society for Antimicrobial Chemotherapy
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Original article
1 Antibiotic Resistance Monitoring & Reference Laboratory,
Health Protection Agency, Specialist & Reference Microbiology
Division, 61 Colindale Avenue, London NW9 5HT, UK
* Corresponding author. E-mail: Nicola.Potz{at}hpa.org.uk..
Received 19 December 2003
; 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
Reliability of routine disc susceptibility testing
by the British Society
for Antimicrobial Chemotherapy (BSAC) method
1
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