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.
Each clinician and laboratory worker must understand the exact definition and clinical significance of these categories.
- 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.
- Intermediate: It is applicable to strains 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 (e.g., the urinary tract) or if the high concentration of the antibiotic is used because of its low toxicity.
- The antimicrobial agent may still be effective against the tested isolate but possibly less so than against a susceptible isolate.
- Resistant. If an organism is resistant to particular antibiotic; that organism 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.