Antibiotic susceptibility testing: caeci caecos ducentes?

Antibiotic susceptibility testing is frequently conducted by diagnostic laboratories and some clinicians rely on the information which such testing provides. In this review of the procedure the rationale for interpretation of the agar diffusion test is outlined and the factors which affect determination of zone size and minimum inhibitory concentration (MIC) discussed. The relationship between zone size and MIC is considered, as well as the clinical relevance of MIC breakpoints. Against this background we assess the value of disc diffusion tests for veterinary practice.