Determining the susceptibility of isolated pathogens to appropriate antimicrobial agents is an important function of clinical microbiology laboratories. The development and spread of resistant bacteria, including nosocomial pathogens, as well as the increasing numbers of immunocompromised patients and new antimicrobial agents, emphasize the importance of rapid and accurate susceptibility testing of isolates from critically ill patients.
In vivo susceptibility is influenced by several factors: host defense mechanisms, the concentrations of antimicrobial agents at the site of infection, the natural course, nature, and severity of the infection, any delay in starting therapy, and the effects of other therapeutic measures such as surgery. In vitro susceptibility depends on the organism, growth medium, atmosphere, inoculum size, and length and temperature of incubation; because these factors have been standardized for common rapidly growing bacteria, the outcome of infection is mainly affected by in vivo factors.
Organisms are susceptible to an antimicrobial agent if they are inhibited in vitro by a concentration of the agent that is lower than serum concentrations achievable with the usual drug dosage (.NMio.n.al.C^ Resistant organisms are not inhibited or are inhibited only at concentrations above attainable levels. Susceptibility is expressed as the minimum inhibitory concentration (MIC) of an antimicrobial agent required to inhibit growth of a defined population of organisms or as categories based on antimicrobial levels in various body sites. All susceptibility tests are based directly or indirectly on MIC determination and require strict standardization and quality control.
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