Antimicrobial Prescribing Myths

1. The Concept of the Antibiotic Course

As a new grad, I remember being told that treating a particular infection required a 10-day course of antimicrobials because I suggested that I was going to give 7 days. This didn't really make sense to me at the time, it wasn't a particularly severe infection, so 10 days felt excessive. A quick search through VIN brought up various discussions on the subject with vets giving anywhere from 5-10 days of different antimicrobials for the same problem, all claiming successful treatment.

In short, the concept of the antimicrobial 'course' is nonsense. The fact of the matter is, antimicrobial therapy is there to treat an infection. The duration of an infection will vary between patients. Therefore, the duration of antimicrobial treatment should vary too, continuing until a clinical response has been observed. This will require monitoring and stopping or continuing therapy as needed; however, I do appreciate this may be difficult as most of our work is done on an outpatient basis.

2. You Must Complete the Course

Following on from the point above, I'm sure many of you have come across the idea that finishing an antimicrobial course is essential to make sure you kill all the pathogens so no remaining ones develop resistance.

Unfortunately, this is outdated. While the above sounds sensible, it ignores the microbiome. Not only are you trying to kill pathogens with antimicrobials, you are exposing the commensal bacteria in the whole body. Any exposure will promote resistance developing in some bacteria, so the key idea is to reduce exposure to antimicrobials full-stop. This means minimising antimicrobial usage and, in general, there is a trend towards shorter courses in human medicine, and I see no reason why we shouldn't follow suit. Just remember to remind your clients to drop off unused antimicrobials at your practice or a pharmacy for proper disposal.

3. Intravenous Antimicrobials are Better Than Oral

Intravenous (IV) antimicrobials reach plasma concentrations quickly and aren't affected by first-pass metabolism, therefore reaching higher concentrations overall. Therefore, it really does seem that IV drugs will be more effective than those given per os (PO).

Once again, this is another area where evidence-based medicine disagrees with the basic principles. In humans, no differences in treatment outcomes - and even some possible improved outcomes with PO treatment - were observed in cases of osteomyelitis, bacteraemia, endocarditis and community acquired pneumonia when using antimicrobials PO compared to prolonged IV treatment.

Furthermore, PO antimicrobials are often associated with less adverse events and result in shorter hospital stays.

Ideally, we would have good clinical trials for high-stakes infections. However, a general rule of thumb would be that oral therapy should be considered in patients that will tolerate and absorb oral medications when they are haemodynamically stable with adequate source control. Of course, the oral antibiotic chosen should reach high concentrations at the relevant site of infection. This will likely limit IV antimicrobials for diseases where intravenous distribution is essential (e.g. sepsis) or PO is not possible due to severe disease, patient mental status or temperament.

Finally, during our patients hospital stay, regular consideration should be given to IV to oral switching where possible and removal of the intravenous catheter if it is not in use to prevent catheter-related complications.

4. Bactericidial Is Better Than Bacteriostatic

The idea that bactericidal antibiotics kill bacteria whereas bacteriostatic just stop them growing is a common misconception. These terms are actually specific to the in-vitro effect of an antibiotic on bacterial growth. For various reasons, in-vitro testing does not always correlate to in-vivo outcomes. A large number of human trials have not proven any better outcomes for bactericidal antibiotics, and some trials which at least found a difference in outcome actually favoured the bacteriostatic choice!

5. Combination Therapy Must Be Better

This myth has a lot of complexity as there truly are some indications where combination in therapy is indicated. However, as a general rule, combination therapy (using multiple antibiotics concurrently) involves a greater risk of adverse effects and there are limited cases where it will be beneficial if you know the causative organism. Of course, if you don't know the organism and one antibiotic is not broad enough for the most common pathogens, combination therapy would be a good choice. Having said that, that is not a reason for unnecessary broad-spectrum coverage (sometimes referred to as "four quadrant coverage"). We have some idea of the causative organisms in most infections nowadays, and therefore we should limit our antimicrobial spectrum to these where possible.

• Bibliography

  Antibiotic courses

  • Llewelyn M J, Fitzpatrick J M, Darwin E, Tonkin-Crine S, Gorton C, Paul J et al. The antibiotic course has had its day. British Medical Journal, 2017; 358 :j3418 doi:10.1136/bmj.j3418

  • Spellberg B, Rice LB. Duration of Antibiotic Therapy: Shorter Is Better. Annals of Internal Medicine, 2019;171(3):210-211. doi:10.7326/M19-1509

  IV vs PO

  • Wald-Dickler, N. et al. Oral is the new IV. challenging decades of blood and Bone Infection dogma: A systematic review, The American Journal of Medicine, 2022; 135(3). doi:10.1016/j.amjmed.2021.10.007

  • Lee, K. and Mercuro, N. If The Gut Works, Use It! Five Important Considerations When Switching From IV To PO Antibiotics [Internet]. IDstewardship, [Accessed July 2024]. Available from: https://www.idstewardship.com/gut-works-use-five-important-considerations-switching-iv-po-antibiotics/

  • Kaal AG, Roos R, de Jong P, et al. Oral versus intravenous antibiotic treatment of moderate-to-severe community-acquired pneumonia: a propensity score matched study. Scientific Reports. 2024;14(1):8271 doi:10.1038/s41598-024-59026-2

  • Atkinson M, Lakhanpaul M, Smyth A, Vyas H, Weston V, Sithole J, et al. Comparison of oral amoxicillin and intravenous benzyl penicillin for community acquired pneumonia in children (PIVOT trial): a multicentre pragmatic randomised controlled equivalence trial. Thorax, 2007 62(12):1102-6. doi: 10.1136/thx.2006.074906

  Bactericidal vs bacteriostatic

  • Noah Wald-Dickler, Paul Holtom, Brad Spellberg. Busting the Myth of “Static vs Cidal”: A Systemic Literature Review, Clinical Infectious Diseases, 2018; 66(9):1470–1474, doi.org/10.1093/cid/cix1127

  • Spellberg B. 17 - Principles of Anti-infective Therapy. John E. Bennett, Raphael Dolin, Martin J. Blaser. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases (Ninth Edition). W.B. Saunders. 2019.

  • Johnson, M.D. et al. ‘Top myths of diagnosis and management of infectious diseases in hospital medicine’, The American Journal of Medicine, 2022;135(7):828–835. doi:10.1016/j.amjmed.2022.03.019

  • bradspellberg.com [Internet] Spellberg, B. [Accessed July 2024]