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Antimicrobial therapy of infections is arguably the most important scientific development of 20th-century medicine. The rapid evolution of antimicrobial discovery and development in the mid-20th century has forever changed the field of medicine. Today, antimicrobials reduce the morbidity and mortality due to infectious disease and contribute significantly to the quality of life of many people.

The decision-making process for choosing an appropriate antimicrobial agent is summarized in Table 39–1. Accurate clinical diagnosis is based on the patient’s history, physical examination, and initial laboratory tests. This leads to a consideration of organisms commonly associated with the clinical condition and the pattern of antimicrobial susceptibility usually associated with these organisms. Susceptibility patterns may vary regionally. Whenever an antimicrobial is started, how to stop or narrow the antimicrobial should be considered and “end points” established; this is greatly aided by obtaining appropriate microbiologic samples. As the clinical picture evolves and microbiologic data are acquired, antimicrobial therapy should be continuously reevaluated, tailored, and the duration of therapy established, noting that longer durations are not necessarily better.

Table 39–1.aSteps in decision making for use of antimicrobial agents.

In considering the possible causative pathogens, age, immune status, past microbiology, and exposure history are important considerations. For example, neonates are generally predisposed to infection with Escherichia coli, and group B streptococcus. As age increases, infections with Streptococcus pneumoniae and Staphylococcus aureus increase. Immune deficiency increases the number and types of pathogens to be considered, making empiric treatment challenging. Children with chronic illness may have a microbiologic history that should be considered when hospitalized with a suspected infection. Clues that may be obtained from the history include exposure to ill family ...

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