Journal of Antimicrobial Chemotherapy (1999) 43, 314-315
© 1999 The British Society for Antimicrobial Chemotherapy
Correspondence |
Sensitivity testing of Pseudomonas aeruginosa to ciprofloxacin: comparison of the modified Stokes' method with MIC results obtained by the Etest
J Antimicrob Chemother1999; 43: 314–315
a Public Health Laboratory, Newcastle General Hospital, Newcastle upon Tyne NE4 6BE; b Department of Microbiology, Hexham General Hospital, Hexham, Northumberland NE46 1QJ, UK
Sir,
Gosden et al. reported on the comparison of the modified Stokes' method with the results of MIC testing by an agar incorporation method. 1 We would like to report our experience of a similar comparison with Pseudomonas aeruginosa isolates, comparing the modified Stokes' method with the MIC result obtained by the Etest. Evaluation of the Etest for antimicrobial sensitivity testing of P. aeruginosa isolates to ciprofloxacin has been shown to be reliable compared with the agar dilution method. 2,3 One hundred and four isolates of P. aeruginosa (47 sensitive, four intermediate, and 53 strains resistant to ciprofloxacin as defined by the modified Stokes' method) from hospital in-patients and out-patients and from general practitioners were tested. The disc diffusion test with a ciprofloxacin 1 µg disc was carried out using Diagnostic Sensitivity Test (DST) agar (Mast Diagnostics, Merseyside, UK) and the rotary modified Stokes' method with the control organism (P. aeruginosa NCTC 10662) on the outside and the test organism in the centre.
Interpretation of the results of sensitivity testing for the Stoke's method used the British
Society for Antimicrobial Chemotherapy (BSAC) criteria, i.e. an isolate was reported as sensitive
if the zone radius was equal to, wider than, or not >7 mm smaller than the control;
intermediate if the zone radius was >2 mm but smaller than the control by 7 mm; and resistant
if the zone radius was 
2 mm.
4 The Etest (AB Biodisk, Solna, Sweden) was performed on
Iso-Sensitest agar (Unipath Ltd, Basingstoke, UK) using the manufacturer's instructions.
A
comparison was made between the MIC result obtained by the Etest
and the sensitivity result obtained from the Stoke's method. The results are given in the
Table. No major errors were identified, i.e. no resistant strains were
reported as sensitive. However, one minor error (i.e. sensitive strain reported as resistant) was
noted using the higher BSAC set
of breakpoints (1, >1–4 and >4 mg/L) and no errors using the lower BSAC set
of breakpoints (0.5, 1–2 and >2 mg/L). A total of 17 isolates had an intermediate
MIC, i.e. >1 and <4 mg/L (low-level resistance). In this group the greatest number of
misclassifications
occurred. This is perhaps to be expected, as P. aeruginosa shows a continuous
distribution of MICs and does not divide into two distinct (resistant and sensitive) populations; a
disc test would therefore not clearly classify isolates in the same way as an MIC test, and
categorization
errors do occur.
5 Overreporting of ciprofloxacin resistance for P.
aeruginosa by the disc method has been previously highlighted as a particular problem by
other workers.
6
|
Our laboratory serves a district general hospital but also houses the Regional Spinal Injuries Unit. P. aeruginosa is a frequent cause of colonization and infection in patients with spinal cord injury and we are therefore particularly interested in ensuring that ciprofloxacin sensitivity to P. aeruginosa isolates is reported accurately. The quinolones are the only effective oral anti-pseudomonal agents available and their use may allow patients who require treatment to return home rather than stay in hospital for intravenous therapy. Routine determination of MICs for all P. aeruginosa isolates in our laboratory would not be feasible. As a result of our study and because of the difficulty in reading small zone sizes, we developed a local protocol in which P. aeruginosa isolates that show a zone of inhibition to ciprofloxacin, but this radius is
3 mm, have an MIC determined using the Etest strip. This has reduced the number of
isolates misclassified. We would agree with Gosden et al.
1 that the modified Stokes' method for sensitivity
testing may show an unacceptable number of major errors for some organisms; however, for P. aeruginosa susceptibility to ciprofloxacin we have found the
modified Stokes' method to be acceptable provided its limitations are appreciated. We will
be interested to compare our previous results with those obtained using the standardized disc
method of sensitivity testing recommendedin the BSAC Summer Newsletter, 1998.
Notes
* Corresponding author. Tel: +44-191-273-8811; Fax: +44-191-226-0365; E-mail: newagall{at}newphls.demon.co.uk 
References
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