infectious diseases
Susceptibility Testing
Qualitative methods
Qualitative methods are less precise than semiquantitative. Results are usually reported as one of the following: Susceptible (S) Intermediate (I) Resistant (R) Some strains that do not have established criteria for resistance may be reported only as susceptible or nonsusceptible. Establishment of which specific drug concentrations represent S, I, and R is based on multiple factors, particularly pharmacokinetic, pharmacodynamic, clinical, and microbiologic data. The commonly useddisk diffusion method(also known as the Kirby-Bauer test) is appropriate for rapidly growing organisms. It places antibiotic-impregnated disks on agar plates inoculated with the test organism. After incubation (typically 16 to 18 hours), the diameter of the zone of inhibition around each disk is measured. Each organism-antibiotic combination has different diameters signifying S, I, or R. 