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Antimicrobials vary in their ability to inhibit or kill different species of bacteria. Antimicrobials that kill many different species of bacteria are called broad-spectrum antimicrobials; whereas, antimicrobials that kill fewer different species of bacteria are called narrow-spectrum antimicrobials. Empirically treating infectious diseases and monitoring therapy requires knowledge of antimicrobial properties, host factors, patient’s normal flora, differentiating infection versus colonization, and understanding clinical presentation and diagnostic tests (microbiologic and nonmicrobiologic laboratory studies). Broad-spectrum antimicrobial coverage increases the likelihood of empirically targeting a causative pathogen; unfortunately, the development of secondary infections caused by selection of antimicrobial-resistant pathogens is a common complication. In addition, adverse events may complicate up to 10% of antimicrobial therapy (adverse event rate is higher for select agents).


Antimicrobial Properties

Drug-specific considerations in antimicrobial therapy include spectrum of activity, pharmacokinetic (PK) and pharmacodynamic properties, adverse effects, drug interactions, and cost.

Spectrum of Activity

Patients who receive initial antimicrobial therapy that provides coverage against the causative pathogen survive at twice the rate of patients who do not receive adequate therapy initially. Because empiric antimicrobial therapy selection is critical to patient outcomes, broad-spectrum antimicrobials are often utilized because it will increase the likelihood of empirically targeting a causative pathogen. However, if all patients receive broad-spectrum antimicrobials, resistance would become an even more difficult problem. Therefore, it is important to understand the difference in antimicrobial spectrum of activity and select agent(s) that target the pathogens most likely causing the infection. Table 20-1 lists the spectrum of activity for select antimicrobials.

TABLE 20-1Drugs of Choice, First Choice, Alternative(s)

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