Journal of Antimicrobial Chemotherapy, Vol 42, 161-169, Copyright © 1998 by The British Society for Antimicrobial Chemotherapy
PE Gosden, JM Andrews, KE Bowker, HA Holt, AP MacGowan, DS Reeves, J Sunderland and R Wise
The majority of clinical microbiology laboratories in the UK use
comparative disc diffusion methods based on the Stokes' method to determine
antibiotic susceptibility. The technical validity of the results obtained
from the modified Stokes' method of disc testing and how they relate to MIC
data are not known. We studied susceptibility testing using a modified
Stokes' disc diffusion method for a wide range of clinical isolates against
which MICs had been determined by collaborators not involved with the disc
testing evaluation. Results indicated that for 1301 organism-antibiotic
combinations the number of major errors (where resistant strains were
reported as sensitive) was 21/468 (4.4%) and the number of minor errors
(where sensitive strains were reported as resistant) was 14/713 (1.9%)
using ciprofloxacin breakpoints of 0.5 and 2 mg/L. There was good
correlation between the disc susceptibility test and the MIC for 119
isolates of Enterobacteriaceae tested with the exception of Serratia spp.
Excluding Serratia spp. the number of major errors for Enterobacteriaceae
was 1/200 (0.5%). Data revealed 2/25 (8%) major errors for Pseudomonas
aeruginosa and 1/45 (2.2%) for Acinetobacter spp. Haemophilus influenzae
showed a number of unexpected categorization errors. The modified Stokes'
method performed accurately for Staphylococcus aureus and
coagulase-negative staphylococci when tested for susceptibility to
gentamicin, erythromycin, teicoplanin and vancomycin. No major errors were
reported for Streptococcus pneumoniae and beta-haemolytic streptococci.
Problems occurred with the detection of antibiotic resistance in
Enterococcus spp. Major errors were seen for ampicillin (2/12 strains),
teicoplanin (5/6 strains) and vancomycin (5/13 strains) using a 30 microg
disc but only 1/13 strains using a 5 microg disc. Overall, from our data,
the modified Stokes' disc diffusion antibiotic susceptibility test showed
an unacceptable number of major errors but an acceptable number of minor
errors.
ORIGINAL ARTICLES
Comparison of the modified Stokes' method of susceptibility testing with results obtained using MIC methods and British Society of Antimicrobial Chemotherapy breakpoints
Department of Medical Microbiology, Southmead Health Services NHS Trust, Westbury-on-Trym, Bristol, UK.
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