Overview of interpreting microbiological testing results

The Antimicrobial Clinical Care Standard recommends using microbiological test results to guide prescribingAustralian Commission on Safety and Quality in Health Care (ACSQHC), 2020. The ability to interpret these microbiological results is essential to appropriate antimicrobial prescribing.

If a microbiological sample is considered to have significant pathogen growth by the laboratory, susceptibility testing is routinely performed. Interpretation of susceptibility results guides antimicrobial selection and directed therapy. In Australia, there are 3 main standardised methods for interpretation of antimicrobial susceptibility testing that are used by laboratories:

• European Committee on Antimicrobial Susceptibility Testing (EUCAST)
• Clinical and Laboratory Standards Institute (CLSI)
• Calibrated Dichotomous Sensitivity (CDS Method).

Categorisation of an organism as resistant or susceptible requires the application of ‘breakpoints’. A breakpoint is an agreed concentration (mg/L) of an antimicrobial that is used to define whether a species of bacteria is susceptible or resistant to that antimicrobial. Breakpoints are usually set by international organisations (eg EUCAST, CLSI). Breakpoints may differ between the 3 methods.

The minimum inhibitory concentration (MIC) is a measure of the lowest concentration of the drug required to stop the visible growth of a specific organism in vitro. The breakpoint is compared to the MIC – if the MIC of the tested organism is less than or equal to the predetermined breakpoint, the organism is only considered susceptible to the antibiotic if a certain dose is used. If the MIC is greater than the predetermined breakpoint, the organism is considered resistant to the antibiotic.

Liaise with a clinical microbiologist or infectious diseases physician in settings of complex infection (eg where there may be difficulty attaining the required tissue levels, a large dose of drug is required to overcome the MIC).

Standard doses of the drug may be effective at different breakpoints if the antimicrobial is naturally concentrated at the site of infection (eg the urinary tract).

The organisms are tested against antimicrobials using a variety of techniques to provide clinical ‘breakpoints’ for interpretation. The techniques that are commonly used include disc diffusion, E test strips for MIC and automated systems for broth microdilution (eg Vitek, Phoenix).