Antimicrobial susceptibility testing (AST)

Antimicrobial susceptibility testing In the clinical setting, the goal of antimicrobial susceptibility testing is to help clinicians choose optimal antimicrobial therapy. 2 The decision to undertake culture and susceptibility testing depends on the site of infection, state of the patient (otherwise healthy vs.critically ill), prior history of infections and antimicrobial use, co-morbidities and underlying disease, and the predictability of the susceptibility patterns of the most likely pathogen(s). Antimicrobial susceptibility tests yield either categorical (susceptible, intermediate, or resistant) or quantitative (minimum inhibitory concentration: MIC) data that can be categorically interpreted. Testing methods can be divided into two distinct categories, diffusion (disk diffusion: Bauer-Kirby procedure or quantitative: gradient strip) and dilution based. Dilutional susceptibility testing can be done using either broth or agar media and yields quantitative (MIC) data. Doubling dilutions of antimicrobial (. . . 0.12 μ g/ml, 0.25 μg/ml, 0.5 μg/ml, 1 μg/ml, 2 μg/ml . . .) are tested. An antimicrobial free control plate or broth must always be included. The lowest concentration without bacterial growth defines the MIC, except for the sulfonamides and trimethoprim, where an 80% reduction in growth compared to the control constitutes inhibition. 

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