General principles and common errors of antibiotic use

This is essentially a summary of the first half of Jeffrey  Lipman's chapter for Oh's Manual. He offers a point-form guide to the use of antibiotics, which seems well suited to the easy generation of SAQs. In order to simplify panicked last-minute cramming, this already laconic list of instructions has been abbreviated yet further. But if brevity is not your thing, you may revel in the excess of the Required Reading section for Infectious Diseases Antibiotics and Sepsis, where antibiotic use is serenaded in the following associated chapters:

General principles of antibiotic use

  • Send cultures before you start the antibiotics
  • Take two sets of cultures, not from a line.
  • Timing with fever is not critical.
  • Do not delay the administration of antibiotics
  • Use empirical therapy first; narrow the spectrum later
  • Use a proper dose first up. No point under-doing your patients.
  • Where possible, use monotherapy (it reduces cost and toxicity).
  • If the microbiology suggests reduced susceptibility, think: are the antibiotics working clinically? Is there direct bedside evidence that they are working? if the answer is yes, then you should continue them in spite of laboratory evidence. In vitro sensitivity does not predict in vivo effect.
  • A shorter course is probably as good as a standard 2-week course
  • Infectious diseases specialists should be involved in managing serious infections.
  • Know your antimicrobial pharmacokinetics and pharmacodynamics; consider tissue penetration and dose adjustment to correct for altered clearance.
  • Monitor the monitorable levels.
  • Limit "prophylactic" use to appropriate situations.
  • Consider non-infective causes of inflammation (it's not always sepsis)
  • Give a strong emhasis to infection control policies.

Common errors of antibiotic use

  • Antibiotics given before cutures are taken
  • Poor quality blood culture collection
  • Ridiculously long courses of antibiotics
  • Wildly erratic changes of antibiotics ("antibiotic surfing") when the sepsis is not improving
  • Inadequate doses
  • Poor choice of empirical antibiotics, failing to account for resident flora
  • Failure to predict toxicity or account for interactions
  • Inappropriate use of antibiotic polypharmacy

Factors which should influence the choice of antibiotic agent

This issue has come up in Question 27 from the first paper of 2015, and then again in Question 5 from the first paper of 2021. It was a very general question ("Discuss the factors that may affect your choice of antimicrobial agent in a critically ill septic patient, giving examples where relevant") and the college were very disappointed with the pass rate (23%). The examiners complained that the trainees gave "superficial answers", including key phrases (eg. "time-dependent killing") without demonstration of understanding. In all fairness, ICU is a business of offering specific answers for specific problems, and the candidates probably do much better with direct knowledge-based questions, eg . "how to kill this Vibrio vulnificus?", or "what is time-dependent killing"? Looking for answers in Oh's Manual also proved fruitless, as even the all-encompassing Lipman chapter ("Principles  of  antibiotic  use", p. 738) is not as broad as the college answer.

A Google search for "factors which influence antibiotic choices" comes up with useless Medscape snippets and studies which were published in1950. It is impossible to say precisely where the college got their answer from, probably because the topic is so diffuse that no author would publish a article about it specifically. Fortunately, a good review from the Mayo Clinic Proceedings is available ( Surbhi et al, 2011). Meat from this article has been grafted to the disarticulated carcass of the college answer for Question 27, forming the table offered below. For rapid revision, the long form of the table can be distilled into a short series of points.

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

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

  • 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

  • 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

  • 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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