New BSAC sensitivity testing guidelines

J Antimicrob Chemother 1999; 44: 850

J. M. T. Hamilton-Miller

Department of Medical Microbiology, Royal Free and University College Medical School, London NW3, UK

Sir,

The recent publications from the British Society for Anti-microbial Chemotherapy (BSAC) Working Party on Sensitivity Testing, BSAC Standardized Disc Sensitivity Testing Method1 and Antimicrobial Sensitivity Testing Guidelines,2 provide clear and specific instructions which will enable these tests to be carried out and interpreted in a uniform manner. The members of the Working Party are to be congratulated on their hard work and dedication. I would be glad of the opportunity to make some comments regarding these papers.

Firstly, it might prove difficult, particularly for those outside the UK, to obtain copies of the guidelines, as neither publication has an ISSN and, thus, might be classed bibliographically as ephemeral.

Secondly, while the imperative to publish the new guidelines for disc sensitivity testing at the earliest opportunity is understandable, this has created dichotomies between the disc diffusion method and other sensitivity testing methods in which solid media are used. For example, if the breakpoint method is employed, the ‘old’ guidelines,3 which stipulate the use of lysed blood, would apply, whereas the ‘new’ guidelines specify that whole blood should be used.

Thirdly, it would have been helpful if the Working Party had given their reasons for replacing lysed blood with whole blood. In my experience, the use of the latter can give rise to problems, e.g., the zones of {alpha}-haemolysis produced by pneumococci can easily be mistaken for zones of inhibition—confusion which does not occur with lysed blood.

Fourthly, another revised recommendation for which an explanation would have been helpful concerns the mandatory use of an atmosphere containing CO2 when determining the susceptibilities of pneumococci, meningococci, gonococci and Haemophilus influenzae; the 1991 guidelines recommend that such an atmosphere should be used only for "CO2-demanding organisms". the 1998 guidelines more closely resemble those of the National Committee for Clinical Laboratory Standards (NCCLS),4 which recommends that all streptococci be incubated in a CO2-containing atmosphere, than previous guidelines. However, they add to the aforementioned dichotomy between the old and new methodologies and increases the likelihood of obtaining ‘false resistance’ results, especially with macrolides.5 Most pneumococci and meningococci grow on IsoSensitest agar supplemented with horse blood in air. Moreoever, it is generally accepted that the presence of CO2 in the atmosphere is a confounding factor in susceptibility testing and that its use should therefore be restricted to those situations in which it is absolutely necessary.

It is perhaps also worth considering the situation regarding susceptibility testing guidelines from a broader perspective. The Working Party has stated that, if the recommended methodology was adopted across the UK, the standard nationally would be higher and the problem of not being able to compare the results obtained in one centre with those obtained in another would be overcome. However, the new guidelines represent a step away from the most desirable goal of all—namely, global harmonization. Most microbiologists would agree that it is necessary to comply with NCCLS guidelines, although they are, in many respects, far from ideal, particularly when a new antibiotic is being tested and publication in a USA-based journal is contemplated. It would, therefore, be a bold step, which one hopes would encourage other nations to follow, if, when the next revision of the guidelines is undertaken, the BSAC adopted those of the NCCLS in the spirit of true international standardization. Such a stance might even persuade the NCCLS to act in the same spirit and to accommodate the views of the BSAC Working Party and other similar bodies by modifying some of its recommendations.

Notes

Corresponding author. Tel: +44-1717-940500; Fax: +44-1714-359694; E-mail: j.hamilton-m iller{at}rfhsm.ac.uk

References

1 . Working Party on Antibiotic Sensitivity Testing of the British Society for Antimicrobial Chemotherapy. (1998). BSAC standardized disc sensitivity testing method. Newsletter of the British Society for Antimicrobial Chemotherapy, Summer 1998, pp. 1–30.

2 . Working Party on Antibiotic Sensitivity Testing of the British Society for Antimicrobial Chemotherapy. (1998). Antimicrobial Sensitivity Testing Guidelines.British Society for Antimicrobial Chemotherapy, Birmingham, UK.

3 . Working Party on Antibiotic Sensitivity Testing of the British Society for Antimicrobial Chemotherapy. (1991). A guide to sensitivity testing. Journal of Antimicrobial Chemotherapy 27, Suppl. D, 1–50.[ISI][Medline]

4 . National Committee for Clinical Laboratory Standards. (1997). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically—Fourth Edition: Approved Standard M7-A4. NCCLS, Wayne, PA.

5 . Johnson, J., Bouchillon, S. & Pontani, D. (1999). The effect of carbon dioxide on susceptibility testing of azithromycin, clarithromycin and roxithromycin against clinical isolates of Streptococcus pneumoniae and Streptococcus pyogenes by broth microdilution and the Etest: Artemis Project—first phase study. Clinical Microbiology and Infection 5, 327–30.[Medline]





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