Antimicrobial susceptibility testing (AST) is laboratory testing used to assess how a cultured microorganism responds to antimicrobial agents. After a clinically relevant bacterium or other organism is isolated, the laboratory may test selected drugs to help classify the organism as susceptible or resistant according to standard breakpoints. Common methods include disc diffusion and minimum inhibitory concentration testing such as broth microdilution. AST supports antimicrobial selection and resistance surveillance, but the laboratory result must still be interpreted with the infection site, drug exposure and clinical context.
What Is Antimicrobial Susceptibility Testing?
Antimicrobial susceptibility testing asks a practical laboratory question:
Is this microorganism inhibited by antimicrobial concentrations associated with an effective treatment regimen?
For bacterial infections, AST is often referred to informally as antibiotic sensitivity testing.
The test is generally performed after a viable organism has been isolated in culture, although modern laboratories may also use validated rapid or molecular methods for selected resistance mechanisms.
Traditional phenotypic AST exposes the organism to antimicrobial agents and measures growth or inhibition.
The final report converts this laboratory response into standard interpretive categories.
Why Is AST Performed After a Positive Culture?
A culture answers “what grew?” or at least identifies the clinically relevant organism.
AST answers a different question: which antimicrobial agents are likely to have useful activity against that isolate under defined conditions?
This distinction matters because two isolates of the same bacterial species can have different resistance profiles.
For example, one isolate may remain susceptible to a commonly used antibiotic while another has acquired a resistance mechanism.
AST helps move treatment from broad empirical coverage toward more targeted antimicrobial use when clinically appropriate.
For the culture process itself, see what is microbiology and blood cultures explained.
How Does Disc Diffusion Work?
Disc diffusion is one of the most familiar AST methods.
A standardised suspension of the organism is spread over an agar plate. Small paper discs containing defined amounts of antimicrobial agents are placed on the surface.
As the organism grows, the antimicrobial diffuses outward from the disc.
If the organism is inhibited, a clear zone of inhibition forms around the disc.
The laboratory measures the zone diameter and compares it with validated breakpoint tables.
EUCAST provides standardised disc-diffusion methods and quality-control guidance used across many European and UK laboratories.
A large zone does not simply mean “better antibiotic”. Interpretation depends on the particular organism-drug combination and breakpoint.
What Is the Minimum Inhibitory Concentration?
The minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial that prevents visible growth under standardised test conditions.
MICs can be measured using methods such as broth microdilution.
In broth microdilution, the organism is exposed to a series of antimicrobial concentrations in liquid medium. The laboratory identifies the lowest concentration where visible growth is inhibited.
EUCAST describes broth microdilution as the reference phenotypic method underpinning susceptibility testing for many organisms.
An MIC is a laboratory measurement, not a dose recommendation by itself. It has to be interpreted against clinical breakpoints.
What Are Clinical Breakpoints?
A breakpoint is a threshold used to translate an MIC or zone diameter into an interpretive category.
Breakpoints consider more than whether a drug inhibits an organism in a dish. Their development can include:
- MIC distributions
- resistance mechanisms
- pharmacokinetics
- pharmacodynamics
- dosing
- exposure at the infection site
- clinical outcome evidence
This is why “susceptible” is a standardised interpretation rather than simply a visual judgement.
What Do S, I and R Mean?
Under current EUCAST definitions:
S — Susceptible, standard dosing regimen
There is a high likelihood of therapeutic success using a standard dosing regimen for the agent.
I — Susceptible, increased exposure
This category does not simply mean “intermediate” in the old sense.
EUCAST defines I as susceptible when increased exposure can be achieved, for example by modifying dose, dosing interval, infusion strategy or because high drug concentrations occur at the infection site.
R — Resistant
There is a high likelihood of therapeutic failure even when exposure is increased.
This terminology matters. Writing “I = resistant-ish” or “I = uncertain” misrepresents the current EUCAST definition.
Why Does Increased Exposure Matter?
Antimicrobial activity in the body depends partly on drug exposure.
Exposure is influenced by:
- dose
- dosing interval
- infusion time
- absorption
- distribution
- kidney or liver clearance
- concentration achieved at the infection site
A bacterium categorised as I may still be treatable with the agent when the recommended increased-exposure regimen is used.
Clinical prescribing decisions belong to healthcare professionals who can integrate susceptibility, patient factors and dosing guidance.
Does “Sensitive” Mean the Antibiotic Will Definitely Work?
No.
Laboratory susceptibility increases confidence that an antimicrobial can be effective under the defined dosing conditions, but no in-vitro result guarantees an individual clinical outcome.
Treatment can also be influenced by:
- infection site
- ability of the drug to reach that site
- source control
- immune status
- organ function
- drug allergies
- interactions
- biofilm
- bacterial burden
- adherence
- mixed infection
The modern EUCAST term is susceptible, rather than relying on the informal word “sensitive”.
Does “Resistant” Mean the Bacterium Grows Normally Next to the Drug?
Not necessarily in that simplistic sense.
Resistance is assigned according to validated breakpoints. The organism’s measured MIC or zone diameter falls into a range associated with a high likelihood of therapeutic failure even with increased exposure.
Different mechanisms can cause resistance, including:
- drug-inactivating enzymes
- altered targets
- reduced permeability
- efflux pumps
- metabolic bypass pathways
Some resistance mechanisms affect one drug; others affect whole antimicrobial classes.
What Is the Role of Quality Control?
AST is highly standardised because small technical differences can affect zone diameters and MICs.
Quality-control procedures monitor:
- media performance
- disc potency
- incubation conditions
- inoculum
- reading technique
- analyser performance
- reference control strains
EUCAST publishes current QC tables and method manuals.
This links AST directly with the broader principles described in quality control vs quality assurance in the laboratory.
Why Not Test Every Antibiotic Against Every Isolate?
Laboratories usually test and report a clinically relevant selection.
The panel can depend on:
- organism identity
- specimen type
- infection site
- local formulary
- resistance epidemiology
- stewardship policy
- intrinsic resistance
- specialist guidance
Testing every drug would be inefficient and could produce misleading options.
Selective or cascade reporting may also support antimicrobial stewardship by prioritising narrower or first-line choices where appropriate.
AST and Antimicrobial Resistance
Antimicrobial resistance (AMR) occurs when microorganisms are no longer adequately inhibited or killed by antimicrobial exposures that were previously effective.
AST helps at two levels.
Individual patient care
It supports the selection or refinement of antimicrobial therapy when a relevant organism has been cultured.
Surveillance
Aggregated susceptibility data help laboratories and public-health systems track resistance patterns.
UK Standards for Microbiology Investigations are used to support standardised diagnostic microbiology pathways, and UKHSA reference services contribute to characterisation of unusual or important resistance.
Phenotypic vs Molecular Resistance Testing
Phenotypic AST measures the organism’s actual growth response to antimicrobial exposure.
Molecular tests look for specific genes or mutations associated with resistance.
The two approaches can complement each other.
A molecular test can be rapid but only detects the resistance determinants included in the assay. Phenotypic testing may detect the overall effect of known and unknown mechanisms but usually requires viable growth and additional time.
The best method depends on the organism, clinical urgency and validated laboratory pathway.
What Happens Before AST?
A simplified diagnostic pathway may be:
- specimen collection
- culture
- detection of growth
- organism identification
- susceptibility testing
- validation and reporting
In blood cultures, preliminary identification and rapid resistance markers may sometimes be available before full AST.
For urine or wound cultures, susceptibility testing may be performed when growth is considered clinically significant under local protocols.
What Does an AST Report Look Like?
A report may list:
- organism name
- antimicrobial agents
- S, I or R categories
- MICs for selected drugs
- interpretive comments
- resistance-mechanism comments
- suggested specialist discussion for unusual patterns
Not every result is necessarily reported to the clinician even if it was tested. Laboratories can suppress some results according to stewardship and reporting rules.
Why Clinical Context Still Matters
AST is powerful because it links a specific isolate with drug activity, but it does not answer every treatment question.
A laboratory does not know the entire patient’s history from a plate alone.
Clinical teams may need to integrate:
- whether the culture represents infection or colonisation
- specimen quality
- infection severity
- allergies
- organ function
- pregnancy
- interactions
- site penetration
- source control
That is why susceptibility results inform, rather than replace, clinical judgement.
Key Points
- AST evaluates how a cultured microorganism responds to antimicrobials.
- Disc diffusion uses inhibition-zone diameters.
- MIC methods measure the lowest concentration preventing visible growth.
- EUCAST breakpoints translate measurements into S, I and R categories.
- I currently means susceptible, increased exposure.
- Resistant means a high likelihood of failure even with increased exposure.
- AST supports both patient care and AMR surveillance.
- Results must be interpreted with specimen and clinical context.
References
- EUCAST. Definition of S, I and R. https://www.eucast.org/bacteria/clinical-breakpoints-and-interpretation/definition-of-s-i-and-r/
- EUCAST. Disk Diffusion and Quality Control. https://www.eucast.org/bacteria/methodology-and-instructions/disk-diffusion-and-quality-control/
- EUCAST. MIC Determination. https://www.eucast.org/bacteria/methodology-and-instructions/mic-determination/
- UK Health Security Agency. Standards for microbiology investigations (UK SMI). https://www.gov.uk/government/collections/standards-for-microbiology-investigations-smi
This article is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional about your results.
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Frequently Asked Questions
What is antimicrobial susceptibility testing?
It is laboratory testing that measures how an isolated microorganism responds to selected antimicrobial agents and classifies the result using validated breakpoints.
What is the difference between disc diffusion and an MIC test?
Disc diffusion measures the diameter of growth inhibition around an antimicrobial disc, while an MIC method estimates the lowest antimicrobial concentration that prevents visible growth under standard conditions.
What does susceptible mean on a culture result?
Under EUCAST, S means susceptible at a standard dosing regimen, indicating a high likelihood of therapeutic success when the standard exposure is achieved.
What does I mean on an EUCAST susceptibility report?
I means susceptible, increased exposure. It indicates a high likelihood of therapeutic success when exposure to the antimicrobial is increased appropriately, rather than simply meaning an uncertain middle category.
What does resistant mean?
EUCAST defines R as resistant, meaning there is a high likelihood of therapeutic failure even when antimicrobial exposure is increased.
Can susceptibility testing guarantee that an antibiotic will work?
No. In-vitro susceptibility is important evidence, but clinical outcome also depends on infection site, drug exposure, source control, organ function, immune status and other patient-specific factors.