Interpretation of Susceptibility testing: what does Susceptible, Intermediate and Resistant Mean?

When the result of antimicrobial susceptibility testing is reported to the clinician, pathogenic microorganisms are classified into one of the three categories; “Resistant”, “Intermediate” or “Susceptible”. Other less used categories are “Susceptible-dose dependent” and “Nonsusceptible”. These interpretive criteria are based on MIC or zone diameter values.

Each clinician and laboratory worker must understand the exact definition and clinical significance of these categories.

  1. Susceptible:  This category indicates that the antimicrobial agent in question may be an appropriate choice for treating the infection caused by the bacterial isolate tested.  i.e. the organism is likely to respond to treatment with this drug, at the recommended dosage.
    Bacterial resistance is absent or at a clinically insignificant level.
  2. Intermediate:  It is applicable to those isolates that are “moderately susceptible” to an antibiotic. The intermediate category serves as a buffer zone between susceptible and resistant.
    Intermediate category is used to indicate a number of possibilities including:
    • These antimicrobials can be used in the body sites where it may be concentrated in the focus of infection (eg, quinolones and β-lactam  in urine) or  or when a higher than normal dosage of a drug can be used (eg, β-lactam) because of its low toxicity.
    • The antimicrobial agent may still be effective against the tested isolate but  response rates may be lower than for susceptible isolates
  3. Resistant. If an isolate is resistant to particular antibiotic; it wont be inhibited by the usually achievable concentrations of the agent with normal dosage schedules. This isolate is expected not to respond to a given drug, irrespective of the dosage and of the location of the infection. Antimicrobials of this category are not appropriate choice for treating the infection caused by the bacterial isolate tested. That bacterial isolate is not inhibited by serum-achievable levels of the drug.

Note: For testing the response of staphylococci to benzylpenicillin, only the categories “susceptible” and “resistant” (corresponding to the production of β-lactamase) are recognized.

Other categories

  1. Susceptible-Dose Dependent (SDD):
    “susceptible-dose dependent” is a new category for antibacterial susceptibility testing. If a particular isolate falls under this category, we have to remember that the susceptibility of that isolate depends on the dosing regimen used. Higher doses or more frequent doses or both is used to achieve concentration levels that is likely to be clinically effective against the isolate. While prescribing that antibiotics clinicians should give propoer consideration to the maximum approved dosage regimen.
  2. Nonsusceptible (NS)
    The “nonsusceptible” category is used for isolates for which only a susceptible interpretive criterion has been designated because of the absence or rare occurrence of resistant strains.
    Isolates for which the antimicrobial agent MICs are above or zone diameters below the value indicated for the susceptible breakpoint should be reported as nonsusceptible.

References and further reading: