Antibiotic Susceptibility Testing – Method and Turnaround Time

Method

Antibiotic susceptibility testing is performed in accordance with current EUCAST guidelines. Testing is undertaken once a clinically significant organism has been isolated and identified.

A full panel of antibiotics is normally tested in the laboratory. However, reporting of results is aligned with local antimicrobial stewardship policies, and restricted agents are not routinely released on the initial report. If susceptibility results for additional or restricted antibiotics are required, please contact the laboratory to discuss.

Turnaround Time

In most cases, antimicrobial susceptibility results are available approximately 24 hours after organism identification. This reflects the additional incubation time required to generate and interpret susceptibility data.

Please note that some organisms (e.g. slow-growing or fastidious bacteria) or complex resistance patterns may require longer turnaround times.

Interpretation of Results (S / I / R)

Susceptibility results are reported as:

• S – Susceptible: A high likelihood of therapeutic success at standard dosing.
• I – Susceptible, Increased Exposure: Therapeutic success is likely if exposure to the antimicrobial agent is increased (e.g. higher dose, increased frequency, or optimized pharmacokinetics).
  This reflects the current EUCAST definition and does not indicate reduced activity.
• R – Resistant: A high likelihood of therapeutic failure, even with increased exposure.

Clinical interpretation should take account of the infection site, organism, patient factors, and dosing strategy.

Antibiotic Selection

Clinicians are advised to refer to Trust antimicrobial prescribing guidelines when selecting therapy, including dose and route, particularly where an organism is reported as I (Susceptible, Increased Exposure).

For clinical advice, urgent queries, or requests for additional susceptibility results, please contact the Microbiology laboratory.

Web page (MB/UI/YS-1) updated 28/04/2026