In this article a pharmacist with specialized training in infectious diseases discusses five key areas of study to master in the field of antimicrobial pharmacotherapy, to become an antibiotic ninja.
Authored By: Timothy P. Gauthier, Pharm.D., BCPS, BCIDP
Article posted 11 October 2022
It is commonplace for learners such as students and residents to struggle with infectious diseases pharmacotherapy. The subject involves a large amount of memorization, every rule seems to have at least one exception, and the most common answer to infectious diseases questions is “it depends.”
Which achievement must one unlock to become an antibiotic ninja and develop the clinical reflexes necessary to quickly identify rational pharmacotherapy care plans? Well, there are of course many, but there a few key areas of achievement to focus on. These are discussed here…
Achievement 1: Bugs & Drugs
You do not necessarily need to memorize the antimicrobial spectra pages of the Sanford Guide, but you do need to have a general idea of which drugs cover which bugs. Study resources like this bugs and drugs table can assist with the task and there are many cheat sheets out there too.
Some of the basics include things like:
- Aztreonam and polymyxins are only for Gram negatives
- Glycopeptides (e.g., vancomycin) are only for Gram positives and are commonly used to cover MRSA
- Macrolides, fluoroquinolones, and tetracyclines are go-to drugs for atypical bacteria
- Oxacillin and nafcillin are pretty much only used for MSSA
- Metronidazole and clindamycin are go-to drugs for anaerobes
- Nitrofurantoin has excellent E. coli coverage
- Cefepime and piperacillin-tazobactam are routinely used to cover Pseudomonas
- Daptomycin and linezolid are commonly used drugs when VRE is causing infection
- Cabapenems are go-to drugs for invasive ESBL infections
- All the drugs of choice (e.g., ampicillin for Listeria or Enterococci, SMX/TMP for Stenotrophomonas, doxycycline for Lyme disease, penicillin for syphilis, voriconazole for Aspergillus)
Once you have the fundamentals down, you can then expand to some of the less commonly seen organisms or less clinically used drug-bug combinations employed. I will challenge you to fully unlock this achievement, a good test is to be able to name all of the anti-MRSA and anti-pseudomonas drug from memory. This list can help you practice the task.
Achievement 2: Monitoring & Potential Toxicities
This is important prior to initiation and during a treatment course. It involves not just knowing what to watch out for, but when to look out for it too. For example infusion-mediated reactions from amphotericin B can occur very early on in therapy, while linezolid-induced peripheral neuropathy is associated with use for 4 weeks or more.
Some fundamental monitoring and toxicity points for antibiotics include:
- CPK monitoring with daptomycin for potential rhabdomyolysis
- Renal function monitoring with vancomycin, IV acyclovir, or amphotericin B for nephrotoxicity
- Hepatic function monitoring with oxacillin and nafcillin liver toxicity
- Platelet monitoring with linezolid for potential thrombocytopenia
- Diarrhea from clindamycin
- Nausea/vomiting from tigecycline
A good test of your toxicity knowledge is to list all of the important fluoroquinolone toxicities from memory. You can find them here in detail or here in short summary. In addition, this blog post on antibiotic toxicities from A to Z may be a fun way to review antimicrobial toxicities.
Achievement 3: Applicable Infection Types
It is one thing to know which drugs cover which bugs, but it is another to know which infections the drugs can or cannot be used for.
Of course this is infectious diseases, so it is not quite that simple. Sometimes drugs can be used in combination for a given infection type or organism but not alone for it. For example using ceftraixone plus ampicillin for Enterococcal infective endocarditis is commonplace, whereas you would otherwise never use ceftriaxone alone for any Enterococcal infections.
Again start with the basics and the hard-stops, like:
- Vancomycin PO and fidaxomicin are only for C. difficile
- Nitrofurantoin is only for treating lower UTI/cystitis
- Methenamine is only for UTI prophylaxis
- Daptomycin is not for pneumonia
- Moxifloxacin is generally not for UTIs
- Tigecycline is generally not for bloodstream infections
This may be one of the most difficult achievements to unlock, but the first step towards it is getting down the bugs and drugs. A good way to test your knowledge in this area is the LearnAntibiotics Bugs & Drugs Practice test.
Achievement 4: Routine Antibiotic Doses
Many drugs are manufactured in the doses for which they are commonly used. This is good news for people who have worked in the healthcare environment who need to be aware of routine doses of antibiotics. But also take note that a general rule is if you have to use 3 or more vials or pills to make a dose, you better check the dose to make sure it is correct.
A few routine antibiotic doses to know include:
- Azithromycin as a Zpak at 500 mg PO x1, then 250 mg PO daily x4 days (5 days total treatment)
- Doxycycline 100 mg PO/IV BID
- Metronidazole 500 mg IV/PO BID or TID
- Linezolid 600 mg IV/PO BID
- Daptomycin 6-10 mg/kg IV daily
- Ceftriaxone IV 1 gm daily, 2 gm daily, or 2 gm BID
- Ceftaroline 600 mg IV BID or TID
- Ciprofloxacin 500 mg PO BID or 400 mg IV BID
- Levofloxacin 250 mg or 500 mg or 750 mg IV/PO daily
In knowing common antibiotic doses, renal adjustments are important. So much so that I actually recommend memorizing those that do not require renal adjustment rather than those that do require renal dose adjustment, because you will get to memorize a shorter list! Some antimicrobials that do not require renal function adjustments include:
- Oxacillin/nafcillin
- Ceftriaxone
- Amphotericin B
- Moxifloxacin
- Tigecycline
- Linezolid
Here is a helpful blog post about renal dosing for antibiotics to help review the topic, which by the way should also account for the indication (i.e., penetration to the site of infection). Antibiotic dosing familiarity comes with lots of practice, so expect it will take time to master.
Achievement 5: Contraindications and Drug-Drug Interactions
Contraindications may be stopping points while drug-drug interactions may be more of a traffic light situation depending on the case. Green light go, yellow light proceed but with caution, red light stop. Antimicrobials with many drug-drug interactions should be recognized, including:
- Rifampin via P450
- Azole antifungals (less-so isavuconazole) via P450
- Linezolid with SSRIs
- Ritonavir P450
Contraindications to use include situations such as…
- Itraconazole in CHF patients
- Fluoroquinolones in myasthenia gravis patients
This achievement takes more time to unlock, but at least rifampin and voriconazole for example are drugs to always watch out for with interactions!
Closing Comments
If it is not clear yet, the honest secret to becoming comfortable with complicated infectious diseases cases is simple. It takes lots of time and practice. Having the right resources as you go through your journey is key and I hope you will consider using my platform for this, which is called Learn Antibiotics by IDstewardship.
Good luck!!!
