Clarification on EUCAST susceptibility testing guidance

No disc diffusion criteria for S. aureus against vancomycin/teicoplanin

The disc diffusion method (BSAC or EUCAST) does not accurately identify glycopeptide resistance in Staphylococci. Guidance from the EUCAST is to perform an MIC by broth microdilution, due to variability seen in gradient strips. Results with gradient strip methods may be 0.5-1 two-fold dilution steps higher than the results obtained by broth microdilution. The variability of gradient strip MICs are currently being reviewed by BSAC/EUCAST.

  • BSAC CURRENTLY RECOMMENDS: Perform an MIC test by broth microdilution preferably or await outcome of review.

 
No disc diffusion criteria for S. pneumoniae against cephalosporin

The preferred algorithm for susceptibility testing of cephalosporins in S. pneumoniae is to perform an MIC method following the ß-lactam screen test using an oxacillin (1ug) disc. The flow chart at the bottom of the S. pneumoniae EUCAST breakpoint table should be used.

Most appropriate method for Enterobacteriales or Pseudomonas species against colistin

No current disc method is adequate to determine susceptibility/resistance to colistin in Enteriobacteriales or Pseudomonas species. Please see link for advice on colistin susceptibility testing in Gram negative organisms:

 
No EUCAST disc diffusion criteria or MIC breakpoints for temocillin

There are currently no EUCAST disc diffusion or MIC breakpoints for temocillin. The data required for determining breakpoints is still under investigation by EUCAST; there are complexities surrounding the data, which need to be resolved.

 
BSAC/EUCAST do not have any recommendation as yet but some laboratories have been testing and interpreting using BSAC methods/breakpoints. However, this is not ideal when the majority of disc diffusion testing is performed by the EUCAST method. A few laboratories have been testing with the EUCAST disc diffusion method then interpreting using the breakpoint and zone diameter criteria suggested in a 2013 paper: Vanstone et. al, ‘’ Temocillin disc diffusion susceptibility testing by EUCAST methodology’’ J. Antimicrob Chemother (2013) 68 (11): 2688-2689. Here ≥20mm and ≥12mm correspond to ≤8mg/L and ≤32mg/L MIC breakpoints. This is the criteria used by some automated instruments. BSAC/EUCAST will not endorse these criteria until more data and discussions have taken place.

 
No EUCAST disc diffusion criteria for trimethoprim against H. influenzae, M. catarrhalis or S. aureus

There are no EUCAST zone diameter criteria or MIC breakpoints for trimethoprim against H. influenzae or M. catarrhalis. The EUCAST zone diameter criteria and MIC breakpoints for trimethoprim against S. aureus are for use in non-complicated UTI only.

It is considered that trimethoprim is not the optimal choice in these clinical scenarios, especially as monotherapy; there is no clinical evidence of efficacy. Other therapeutic options should be considered for which disc criteria and MIC breakpoints exist, as per co-trimoxazole.

 
For situations where there is no clinical breakpoint, EUCAST offers advice through a guidance document: Click here