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The discovery and rapid development of targeted antimicrobial agents, beginning in the 1930s, are among the most important scientific developments of 20th-century medicine. These drugs have forever changed the practice of medicine, and antimicrobials remain one of medicine’s most effective and widely used interventions. However, despite the impact of these drugs, choosing an appropriate antimicrobial can be a complex and difficult task. Optimal antimicrobial use requires appreciation of the complicated interactions between host, organism, and drug. This decision-making process, summarized in Table 39–1, begins with an accurate working diagnosis, based on the patient’s history, physical examination, exposure history, and initial laboratory tests. From this foundation, the clinician must consider the most likely organism(s) and that organism’s likely pattern of antimicrobial susceptibility. This information is considered in the context of numerous patient-specific factors, including age, immune status, relevant comorbidities, site of infection, and microbiology of his or her prior infections. Empiric therapy should be changed to definitive therapy as the clinical course evolves and additional laboratory data are available. Obtaining appropriate microbiologic specimens helps facilitate this transition.

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

For example, age impacts the likely pathogens; neonates are generally predisposed to infection with Escherichia coli and group B Streptococcus, while in older children, infections with Streptococcus pneumoniae and Staphylococcus aureus predominate. Children with chronic illness may have significant exposure to antimicrobials, and the effect of this on their microbial flora should be considered. Different exposures, based on environment, travel, diet, animal contact, or ill family members or other close contacts, may suggest a greater likelihood of certain organisms. Another important consideration is the pace and seriousness ...

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