Skip to Main Content


Source: Burgess DS. Antimicrobial Regimen Selection. In: DiPiro, JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM. Pharmacotherapy: A Pathophysiologic Approach. 8 ed. Accessed June 23, 2012.


  • “Empiric” regimen: initiated before offending organism is identified and sometimes prior to documentation of presence of infection.
  • “Definitive” regimen: instituted when causative organism is known.


  • Obtain careful history and physical.
  • Signs and Symptoms
    • Fever
      • Body temperature above normal range of 36.7–37°C (98.1–98.6°F; measured orally)
      • Hallmark of infectious disease
      • May be caused by drugs in absence of infection or other underlying condition.
    • Elevated white blood cell (WBC) count
      • Granulocytes and/or lymphocytes are mobilized to destroy invading microbes.
      • Bacterial infections associated with
        • Elevated granulocyte counts (neutrophils and basophils)
        • Increased band neutrophils in peripheral smear (left-shift)
        • Low neutrophil counts (neutropenia), indicating abnormal response; associated with poor prognosis
      • Tuberculosis, viral, or fungal infections associated with relative lymphocytosis even with normal or slightly elevated total WBC count
    • Local signs
      • Pain and inflammation
        • Swelling
        • Erythema
        • Tenderness
        • Purulent drainage


  • Collect infected body material.
    • Assess with Gram stain.
    • Perform blood cultures and sensitivities.
    • Perform serologic tests for presence of antibodies.
  • Collect suspected fluids or tissues (e.g., spinal fluid in meningitis).
  • Assess inflammation with deep-seated infections by examining tissues or fluids (e.g., examine sputum to assess pneumonia).


  • Factors to consider
    • Severity and acuity of disease
    • Local susceptibility data rather than national compilations
    • Host factors
      • Allergy or history of adverse drug reactions
      • Age of patient
      • Pregnancy
      • Metabolic abnormalities
      • Renal and hepatic function: Adjust dosage with diminished renal and/or hepatic function to avoid drug accumulation.
      • Concomitant drug therapy: Potential for drug interactions (Table 1)
      • Concomitant disease states
    • Drug factors
      • Use generally accepted drugs based on pathogen (Table 2)
      • Consider pharmacokinetic and pharmacodynamic properties of agent
        • Bactericidal effects may be concentration-dependent (aminoglycosides and fluoroquinolones) or time-dependent (β-lactams).
        • Treatment outcome can be predicted by area under concentration-time curve (AUC) and maximal plasma concentration.
        • Duration that drug concentration exceeds minimal inhibitory concentration (MIC) is most important pharmacodynamic relationship for antimicrobials that display time-dependent bactericidal effects.
      • Antibiotic tissue penetration varies with site of infection.
        • Clinically relevant drug concentrations found in blood, urine, synovial fluid, and peritoneal fluid.
    • Combination therapy should be considered to:
      • Broaden the spectrum of coverage for empiric therapy
        • Important when multiple aerobic and anaerobic bacteria are likely to be present (e.g., in intraabdominal and female pelvic infections)
      • Achieve synergistic activity against infecting organism
        • Advantageous for infections caused by gram-negative bacilli in immunosuppressed patients.
        • May produce better results in infections caused by Pseudomonas aeruginosa and certain infections caused by Enterococcus spp.
      • Prevent the emergence of resistance

Table Graphic Jump Location
Table 1. Major Drug Interactions with Antimicrobials

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.


About MyAccess

If your institution subscribes to this resource, and you don't have a MyAccess profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.

Subscription Options

AccessPharmacy Full Site: One-Year Subscription

Connect to the full suite of AccessPharmacy content and resources including 30+ textbooks such as Pharmacotherapy: A Pathophysiologic Approach and Goodman & Gilman's The Pharmacological Basis of Therapeutics, high-quality videos, images, and animations, interactive board review, drug and herb/supplements databases, and more.

$595 USD
Buy Now

Pay Per View: Timed Access to all of AccessPharmacy

24 Hour Subscription $34.95

Buy Now

48 Hour Subscription $54.95

Buy Now

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.