Question 5

Discuss the factors that may affect your choice of antimicrobial agent in a critically ill septic patient, giving examples where relevant.

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College answer

Not available.


The tabulated answer below was largely derived from an article by  Surbhi et al (2011) from the Mayo Clinic Proceedings. Neither this short version nor the long version that follows are suitable for a 10-minute answer, but should be viewed as vague guides.

Factors which influence antibiotic choice
Disease factors Host factors Organism factors Drug factors
  • Travel history
  • Occupation
  • Recreational exposure
  • IVDU
  • Severity of illness, urgency of therapy
  • Reliability of cultures
  • Age
  • Clearance organ function
  • Allergies
  • Immune status, HIV
  • Pregnancy and lactation
  • Source control
  • Susceptibility
  • Empiric vs specific
  • Intra vs. extracellular
  • Duration of therapy
  • Assessment of response
  • Cost
  • Toxicity
  • Bioavaiability
  • Source site penetration
  • Drug synergy
  • Bacteriostatic vs bactericidal
MORE Factors which Influence the Choice of Antibiotic Therapy
Factors Discussion and examples
Disease specifics Travel history
  • Geography of endemic regions (eg. leptospirosis)
  • Known ongoing outbreaks (eg. Ebola, H1N1, MERS)
Occupational exposure
  • Abbatoir workers (Coxiella burnetii)
  • Fisherman (Vibrio vulnificus)
  • Cattle farmers (Brucella sp.)
Recreational exposure
  • IV drug use (endocarditis)
  • Pets or animal exposure (eg. psittacosis or toxoplasma)
  • Bushwalking (eg. tick-borne disease)
  • Alcoholism (prognostic importance in community-acquired pneumonia)
Recent antimicrobial use
  • Was it the right antibiotic? i.e. was the course of antibiotics ineffective because of poor agent choice?
  • Did it select for a specific group of organisms?
  • Prophylaxis vs. endemic pathogens (eg, malaria)
Empiric vs. definitive
  • Are we convinced of the diagnosis?
  • Is there a need to cover broadly?
Urgency and timing
  • Septic patient (every hour delay is associated with a 1% mortality increase)
Reliability of cultures
  • Are we sure we cultured the correct pathogen?
  • Is a polymicrobial infection possible (eg. diabetic foot)?
Host factors Clearance
  • Decreased renal clearance (by renal failure)
  • Increased renal clearance (by dialysis, or in pregnancy)
  • Decreased hepatic clearance (eg. cirrhosis)
  • Exotically altered clearance (eg. plasma exchange, haemoperfusion, adsorption on to ECMO circuit surfaces, and so forth).
  • Paediatric dosing needs to be adjusted to weitght
  • Geriatric dosing needs to account for change in volume of distribution and clearance
Genetic variation
  • Genetic differences in side effects from antibiotics
  • Congenital idiosyncracies preventing the use of certain antibiotics (eg. G6PD deficiency resulting in haemolysis when exposed to dapsone or nitrofurantoin)
  • Hepatic enzyme defects
Pregnancy and lactation
  • Early pregnancy teratogenesis (eg. nitrofurantoin, chloramphenicol, sulfonamides)
  • Late pregnancy teratogensis (eg. tetracyclines)
  • Steroid use
  • Post-splenectomy, unvaccinated (susceptible to encapsulated organisms)
  • Chemotherapy
  • Solid organ or bone marrow transplantation
  • Fatal hypersensitivity reaction vs. some sort of mild scaly rash with a little itching.
Organism factors Susceptibility
  • ESCAPPM, MRO, etc
  • Community prevalence of drug resistance
  • Tendency to develop resistance during treatment
  • Intracellular pathogen vs. extracellular
  • Unusual life cycle (eg. helminthes, malaria) - need to kill the eggs or dormant cocoons or whatnot
Source control
  • Success of therapy overall is largely determined by this
Duration of therapy
  • Short course, eg. in urosepsis
  • Long course, eg. osteomyelitis
Assessment of response
  • To repeat the cultures, or not?
  • Is there a point in monitoring serology?
Drug factors Cost
  • Fluconazole: $57.99 AUD for 28 capsules (200mg)
  • Anidulafungin: ~$ 300 AUD per single 200mg dose.
  • Cost of monitoring the drug levels
  • How much is a life worth? you amoral monsters, etc.
  • Risk vs benefit
  • Some drugs (eg. chloramphenicl) are uniformly  "too toxic for use", as there are less toxic alternatives in almost every situation.
  • Convenience of oral dosing
  • Certainty of IV dosing
  • Altered absorption via GI tract in context of critical illness, shock states, low flow, what have you.
Site penetration
  • Basic chemistry of the drug influences this aspect. Eg:
  • Penetration to the CSF (lipophilicity)
  • Exclusive distribution into the circulating volume, (hydrophilicity, or high serum protein binding)
  • Weird organ preference (eg. the strange affinity of fluoroquinolones for the prostate)
  • Exclusion of a drug from a specific organ (eg. the inactivation of daptomycin by lung surfactant)
Bactericidal vs bacteriostatic
  • Some agents are bacteriostatic against one pathogen and bactericidial against another
  • There may not be any in-vivo difference
Synergistic combination
  • Need for multiple agent therapy (eg. in Pseudomonas)
  • Unquestioned need for synergy (eg. cocktail for TB)
  • Advantage from synergy (eg. ampicillin with gentamicin for enterococci)
  • Need for broad-spectrum coverage (in which case you use multiple agents to start with, and then narrow the spectrum of cover)
  • Need for polymicrobial coverage (eg. surgical triple therapy, or in context of bone marrow transplant)
  • Need to prevent emergence of resistance (eg. the argument offered to defend the use of selective digestive tract decontamination; also, a genuine argument for the use of  rifampicin and fusidic acid together)


Oh's Intensive Care Manual: Chapter 72  (pp. 738)  Principles  of  antibiotic  use  by Jeffrey  Lipman

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