The result of the susceptibility test, as reported to the clinician, is the classification of the microorganism in one of two or more categories of susceptibility. The simplest system comprises only two categories: susceptible and resistant. This classification, although offering many advantages for statistical and epidemiological purposes, is too inflexible for the clinician to use. Therefore, a three-category classification is often adopted. The Kirby-Bauer method and its modifications recognize three categories of susceptibility and it is important that both the clinician and the laboratory worker understand the exact definitions and the clinical significance of these categories.
• Susceptible. An organism is called “susceptible” to a drug when the infection caused by it is likely to respond to treatment with this drug, at the recommended dosage.
• Intermediate susceptibility covers two situations. It is applicable to strains that are “moderately susceptible” to an antibiotic that can be used for treatment at a higher dosage because of its low toxicity or because the antibiotic is concentrated in the focus of infection (e.g., urine).
The classification also applies to strains that show “intermediate susceptibility” to a more toxic antibiotic that cannot be used at a higher dosage. In this situation, the intermediate category serves as a buffer zone between susceptible and resistant.
As most clinicians are not familiar with the subtle, although clinically important, distinction between intermediate and moderate susceptibility, many laboratories use the designation “intermediate” for reporting purposes.
• Resistant. This term implies that the organism is expected not to respond to a given drug, irrespective of the dosage and of the location of the infection.
For testing the response of staphylococci to benzylpenicillin, only the categories “susceptible” and “resistant” (corresponding to the production of β-lactamase) are recognized.
The ultimate decision to use a particular antibiotic, and the dosage to be given, will depend not only on the results of the susceptibility tests, but also on their interpretation by the physician. Other factors, such as pathogenic significance of the microorganism, side-effects and pharmacokinetic properties of the drug, its diffusion in different body sites, and the immune status of the host, will also have to be considered.