Message to users of BSAC disk diffusion method

BSAC to actively support the EUCAST Disk Diffusion Method for Antimicrobial Susceptibility Testing in preference to the current BSAC Disk Diffusion Method

From January 2016, the BSAC Standing Committee for Antimicrobial Susceptibility Testing, with the support of Council, will:

  • Cease active support, maintainance and development of the BSAC disk diffusion method (queries from laboratories that continue to use the BSAC disk diffusion method will be supported during the transition period).
  • Support UK laboratories in changing to the EUCAST (European Committee on Antimicrobial Susceptibility Testing) disk diffusion method should they wish to do this, through increased educational activities.
  • Re-fashion the Residential Workshops to support a wider range of susceptibility testing and resistance detection methods and particularly support those using EUCAST methods.
  • Re-fashion the current “User Days” to cover a wider range of issues in susceptibility testing.
  • Support EUCAST in the further development and maintenance of the EUCAST susceptibility testing methods.
  • Support UK laboratories implementing EUCAST methods and having queries about the methods.


Since it was first developed and published in 2001, the BSAC standardized disk diffusion method of antimicrobial susceptibility testing has been adopted by more than 175 laboratories across the UK. Annual updates have been published since the initial launch and Version 14 of the method was published on the BSAC website in January 2015.

However, over the last five years there have been a number of developments in the field of antimicrobial susceptibility testing which have rightly led to a re-evaluation of the position of the BSAC method.

The BSAC Standing Committee has been instrumental in supporting the development of EUCAST. It signed-up to the EUCAST process for harmonised MIC breakpoint setting and EUCAST breakpoints have been incorporated into the BSAC guidelines.

Although it was not part of the original EUCAST project, a standardised disk diffusion method (based on the Kirby-Bauer method using Mueller-Hinton agar) has been developed, resulting in a choice of two similar standardised disk diffusion methods (BSAC and EUCAST) that are calibrated against EUCAST breakpoints.

The decision to support the EUCAST disk diffusion method in preference to the BSAC disk diffusion method has been taken for a number of reasons:

  • The EUCAST method is a robust and standardised method. It is correlated to MICs performed according to the international standard method for testing antimicrobial susceptibility (ISO20776-1:2006).
  • Many laboratories in the UK have already changed to using the EUCAST disk diffusion method. This leads to confusion between laboratories, particularly when reviewing NEQAS performance as the BSAC and EUCAST methods may perform differently for some challenging organisms.
  • The EUCAST disk diffusion method has been developed to cover more antimicrobial agent/organism combinations than the BSAC disk diffusion method. A few gaps remain (e.g. Neisseria gonorrhoeae testing), but these are being actively developed.
  • The fact that both BSAC and EUCAST methods are now used across the UK raises issues for the Standing Committee in delivery of relevant day-to-day support and also educational meetings and workshops.
  • The EUCAST disk diffusion method is now the standard method used in most European countries and increasingly outside Europe. This means that EUCAST can draw on a wider international pool of experts and laboratories (including those in the UK) for development and support
  • Use of the EUCAST disk diffusion method would improve international standardisation and comparability and support resistance surveillance.
  • EUCAST is recognised by the EMA for the setting of MIC breakpoints for new agents and is increasingly seen by drug developers as the standard-setting organisation for MIC breakpoints and disk diffusion testing.

Following the decision to actively support the EUCAST disk diffusion method and through discussions with UKAS, we would recommend that laboratories move to a supported disk diffusion method as soon as possible.

To view the EUCAST website, click here and for clinical breakpoints, click here

For further information please contact:

Mandy Wootton (Secretary to the Standing Committee)


Robin Howe (Chair of the Standing Committee), via Mandy Wootton

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