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PRINCIPLES OF ANTIMICROBIAL THERAPY
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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, 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 clinical 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, prior antimicrobial exposure, and microbiology of the patient’s prior infections. Different exposures, based on environment, travel, diet, animal contact, or ill close contacts, may suggest a greater likelihood of certain organisms. The pace and severity of the illness are also important. A severe, rapidly progressive illness should be treated initially with broad-spectrum antimicrobials until a specific etiologic diagnosis is made. A mildly ill ambulatory patient should receive treatment with narrow-spectrum antimicrobials, per national guidelines when available. However, decisions of when not to use antimicrobials are equally important, as unnecessary or additional drugs and longer durations may harm patients. A patient unlikely to have an infection, or with an infection unlikely to benefit from antimicrobials (eg, viral respiratory infection), should be spared the added risk of an adverse reaction.
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Once an appropriate initial antimicrobial is chosen, the clinician must consider ...