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:

• Pharmacokinetics of antibiotics in critical illness
• Antibiotic dosing in renal failure
• Antibiotic dosing during dialysis
• Kill characteristics of antibiotic agents
• Causes of antibiotic treatment failure
• The minimum inhibitory concentration (MIC)
• Post-antibiotic effect
• Antibiotic agents classified by mechanism of action
• Specific antibiotic choices for selected pathogens
• Pharmacology of antifungal agents

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