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Antimicrobial Susceptibility Testing


Where possible, antimicrobial susceptibilities are determined and reported in accordance with the European Committee on Antimicrobial Susceptibility Testing (EUCAST). This is a system that has researched and validated the links between the minimum inhibitory concentrations of antibiotics and zone sizes using standard media and methods. Results are then issued as either ‘resistant’, ‘sensitive’ or ‘Dose dependent susceptible’ depending on cut offs also defined by EUCAST. However, some bug-drug combinations, that we consider essential for clinical management, are not included in the data sheets. In these circumstances, we continue to test in-vitro but decisions on cut offs have been taken based on a combination of use of Clinical and Laboratory standards Institute (CLSI) guidelines and clinical experience. We expect these gaps in the EUCAST datasheets to gradually be filled as they do more research.

Note the following bug-drug combinations are not supported by EUCAST methodology:

  • Enterobacteriaceae – Doxycycline (CLSI)
  • Enterobacteriaceae – Chloramphenicol
  • Enterobacteriaceae – Trimethoprim (EUCAST Urinary Breakpoint used)
  • Enterococci – Doxycycline (CLSI)
  • Enterococci – Daptomycin (CLSI)
  • Pseudomonas – Gentamicin (CLSI 2022)
  • Haemophilus – Cefeuroxime (CLSI)
  • Enterococci – Daptomycin (CLSI)
  • Any organism other than E. coli in UTI – Fosfomycin – (CLSI)
  • Burkholderia cepacia complex and Pseudomonas aeruginosa from Cystic Fibrosis (CF) patients: evidence linking in vitro sensitivity with in vivo efficacy is limited.
  • Stenotrophomonas is tested purely using CLSI guidelines
  • Viridans Streptococci (AHS) – Clarithromycin (CLSI)

We currently use a combination of disc diffusion sensitivity testing, E-test strips and also an automated Microbroth dilution sensitivity method called Microscan – it uses EUCAST and occasionally CLSI methodology.

Some more info on ‘Dose dependent susceptible’ category

This has been introduced in 2021 to address those situations where resistance to an antibiotic can be overcome by giving a higher dose of antibiotic. If you see this in a report, then ensure you use the MAXIMUM dose in the BNF or on the Antimicrobial App/Joint formulary (whether it be oral or IV) in order to achieve the maximum likelihood that the antibiotic will be effective.

Clinical Guidance for Dose-Dependant Susceptible (DDS) Results

Useful Links

For patients:

Susceptibility Testing – Lab Tests Online UK

Antibiotics – NHS

For Healthcare Professionals:

For Hospital – please consult Microguide – Antimicrobial Guidance for RDUH

For GPs – please consult North & East Devon Formulary


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