When treating infection in older patients, selection of an antimicrobial regimen often precedes the identification of the causative pathogen and may sometimes be necessary even before the specific site of infection has been established. In these cases, the choice of therapy is typically based on the clinician's estimation of the most likely causative organism as well as properties of the available antimicrobial agents. This method of antimicrobial selection is described as “empiric,” as opposed to “pathogen-directed,” in which the causative organism has already been determined by the results of clinical cultures and antimicrobial susceptibility testing. This chapter addresses the unique challenges of selecting optimal empiric antimicrobial therapy for older patients with known or suspected infection.
To provide both a framework for discussion as well as a practical tool to be applied at the bedside, the process of selecting empiric antimicrobial therapy for older patients may be considered in three steps. First, to determine the potential causative pathogen as well as the appropriateness of initiating antimicrobial therapy, clinical and epidemiological clues are examined in order to establish the most likely site of infection. Next, estimation of the likelihood of antimicrobial resistance, together with consideration of the seriousness of the patient's infection and overall clinical status are used to determine the most appropriate breadth of antimicrobial coverage. Finally, pharmacological issues, including the pharmacokinetics of the available agents, potential toxicities and drug interactions are considered in order to facilitate the final selection of the most appropriate antimicrobial agent or agents. Once empiric therapy is initiated, the patient's clinical progress is closely followed and the antimicrobial regimen is broadened or narrowed according to the results of testing in the clinical microbiology laboratory. This process of choosing empiric therapy is summarized in Table 125-1.
Table 125-1 Rational Stepwise Approach to the Selection of Antimicrobial Therapy for Older Patients |Favorite Table|Download (.pdf)
Table 125-1 Rational Stepwise Approach to the Selection of Antimicrobial Therapy for Older Patients
I. Estimate the most likely causative pathogen if not known
- Review clinical and epidemiological clues
- Identify the most likely site of infection
II. Determine the appropriate breadth of initial antimicrobial therapy
- Estimate the likelihood of antimicrobial resistance
- Consider the severity of patient's condition and comorbid diseases
III. Integrate pharmacological data to select the empiric choice of therapy
- Evaluate pharmacokinetic and pharmacodynamic data
- Consider the risk of drug toxicity and interactions
IV. Monitor clinical course and laboratory data to refine and narrow antimicrobial regimen
- Direct efforts at confirmation of microbiological diagnosis
In the section that follows, this systematic approach to the management of older patients with known or suspected infection is reviewed in greater detail. Thereafter, the strategy is applied to the discussion of specific recommendations for antimicrobial therapy for a variety of infectious syndromes.
Identifying the Causative Pathogen
When evaluating any patient with possible infection, determination ...