Transition to EUCAST
Antibiotic Susceptibility Testing

Why are we changing?

For years, Clinical Labs in VIC/QLD and NSW/ACT have utilised Calibrated Dichotomous Susceptibility testing (CDS). After nearly 50 years, this quick and reliable Australian method is being phased out and will no longer receive updates or support. We sincerely thank the CDS team for their contributions over the years.

What are the changes with EUCAST?

We will primarily use disc diffusion, where pathogenic organisms grow on agar with antimicrobial-impregnated discs. The zone of inhibition of bacterial growth around the discs allows us to categorise susceptibility or resistance.

The main changes are:

EUCAST susceptibility categories:

• Susceptible, standard dosing regimen (reported as “S”): there is a high likelihood of treatment success using a standard dosing regimen of the agent.
• Susceptible, increased exposure (called “I”, but reported as “H”): there is a high likelihood of treatment success with dose optimisation, or due to an antibiotic’s concentration at the site of infection.
• Resistant (reported as “R”): there is a high likelihood of treatment failure, even when there is increased exposure.

The new definitions emphasise the relationship between organism susceptibility and antibiotic exposure at the infection site. The “H” category (called “I” in EUCAST documents) supports use of the antibiotic – i.e., there are two levels of susceptibility and one of resistance.

Regarding dosing, the EUCAST website lists standard and high doses of antibiotics used to define the breakpoints. Some Clinical Labs reports may include specific dosing considerations. The “S” designation is based on the standard dose, while “H” reflects the higher dosage. EUCAST tables should not be solely relied on for clinical dosing; continue using standard evidence-based treatment guidelines like the Australian Therapeutic Guidelines.

Reporting by site and severity of infection

Did you notice in the dosing table (Table 1) that some agents (#) only have a dose listed for uncomplicated urinary tract infections (UTIs)? The following oral antibiotics are recommended only for uncomplicated UTIs:

• Norfloxacin
• Fosfomycin
• Trimethoprim
• Nitrofurantoin

Uncomplicated UTI refers to infections localised to the bladder without anatomical or functional abnormalities or comorbidities. For other urinary tract infections (often termed complicated UTIs), such as acute pyelonephritis and bloodstream infections (excluding severe sepsis), we will provide dosing guidance for oral amoxicillin and amoxicillin-clavulanic acid in interpretative comments.

For some antibiotics (marked * in the table), there is insufficient evidence for standalone use for infections originating outside the urinary tract. Examples include oral amoxicillin-clavulanic acid for diabetic foot infections, and aminoglycosides for systemic Gram negative infections. EUCAST refers to these as “breakpoints in brackets”. On Clinical Labs reports we will report as “S” or “H” with caveats in the comments, indicating that there are no detectable resistance mechanisms, and that the agent may be suitable for use in combination with:

• Another active antibiotic
• Other measures such as source control

Gentamicin susceptibility for Pseudomonas species will no longer be reported; it will be replaced by tobramycin due to its greater potency.

Table 1: Doses used to define EUCAST breakpoints for commonly used antibiotics.

Abbreviations: UTI = urinary tract infection; BD = twice daily / 12-hourly; TDS = three times daily / 8-hourly; QID = four times daily / 6-hourly; NA = not applicable
Notes: * breakpoints in brackets; # breakpoints for uncomplicated UTI only

In some cases, the “best” or most susceptible category for an organism regarding a particular antibiotic is “susceptible increased exposure” (H), indicating higher doses are
always required. This applies to Pseudomonas aeruginosa with piperacillin-tazobactam, ceftazidime, cefepime, and ciprofloxacin, as well as Haemophilus species with amoxicillin. So don’t worry that you’re suddenly seeing lots of “H” results for these situations – it’s not a change in the organism, but a change in our understanding of how to test and treat it.

When there are no breakpoints

For certain antibiotic-organism combinations, there is not yet enough evidence for EUCAST to have set clinical breakpoints. If a reporting pathologist believes a particular antibiotic may be useful in these situations, we will test the minimum inhibitory concentration (MIC) and apply epidemiological and PK-PD data to provide interpretative comments.

For general inquiries, please email a clinical microbiologist in your state. For patient-specific queries, contact a clinical microbiologist through your state’s call centre (1300 134 111).

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