Antimicrobial Susceptibility Testing

Antimicrobial susceptibility testing, or AST, measures how effectively an antimicrobial agent inhibits the growth of a microorganism isolated from a clinical specimen under standardized laboratory conditions. It determines the organism's in vitro susceptibility to particular antibacterial or antifungal drugs, helping clinicians select appropriate therapy.

What the test measures

AST quantifies or estimates microbial growth in the presence of specified antimicrobial agents. It measures the phenotype expressed under test conditions, rather than directly identifying the genetic mechanism of resistance.

• The minimum inhibitory concentration, or MIC, is the lowest tested drug concentration that prevents visible microbial growth.
• In disc diffusion testing, the diameter of the zone of inhibition around an antimicrobial disc is measured in millimetres.

How results are interpreted

The measured MIC or inhibition-zone diameter is compared with standardized clinical breakpoints for the relevant organism-drug combination.

• Results are commonly reported using categories such as susceptible, intermediate and resistant.
• A susceptible result indicates a higher likelihood of therapeutic success when the drug is used with an appropriate dosing regimen.
• A resistant result indicates that treatment failure is likely because the organism is not sufficiently inhibited at clinically achievable exposure.

Clinical value and limitations

AST supports targeted treatment, antimicrobial stewardship and surveillance of antimicrobial resistance. However, it does not by itself guarantee clinical success because outcome also depends on drug concentration at the infection site, dosage, host immunity and infection characteristics.

• AST is performed through standardized methods such as broth dilution, disc diffusion, gradient diffusion and validated automated systems.
• Results must be interpreted alongside organism identification, specimen source and the patient's clinical condition.