Episode 392: How to remember antibiotic spectrum of activity

In this episode, I’ll discuss how to remember antibiotic spectrum of activity.

Knowing the antibiotic spectrum of activity

Early in my career, I asked an ID physician how he kept track of the spectrum of activity for all the antibiotics (Shout out to “Infectious Disease Phil”).  He told me that he first learned the rules of thumb for antibiotic coverage, then he focused on learning the exceptions to the rules.

Here are 5 examples of antibiotic-related rules and their exceptions:

Example 1:

The Rule:

MRSA is covered by vancomycin, linezolid, daptomycin and ceftaroline

The Exceptions:

1. Daptomycin is inactivated by lung surfactant – don’t use in pneumonia
2. Linezolid is bacteriostatic vs MRSA – don’t use in bloodstream infections
3. Ceftaroline is vancomycin and ceftriaxone rolled into one loveable fuzzball

Example 2:

The Rule:

Piperacillin-tazobactam and the carbapenems ertapenem, meropenem, doripenem, and imipenem cover gram positives, gram negatives, and anaerobes

The Exceptions:

1. Ertapenem doesn’t cover pseudomonas
2. Piperacillin-tazobactam doesn’t cover ESBL well enough to be used in severe infections (it probably works for UTI)

Example 3:

The Rule:

Cephalosporins for empiric use in the ICU are cefepime, ceftriaxone, ceftazidime, and ceftaroline.  They cover gram positives and gram negatives

The Exceptions:

1. Only cefepime and ceftazidime for pseudomonas
2. None cover enterococcus (remember this shows up on the gram stain as gram-positive cocci resembling strep!)

Example 4:

The Rule:

Quinolones ciprofloxacin, levofloxacin and moxifloxacin cover gram positives and gram negatives (double check your local resistance patterns!!!)

The Exceptions:

1. Ciprofloxacin is weak against strep pneumo, but causes the least qtc prolongation and probably doesn’t cause torsades (for a great reference on QTc interactions I highly recommend the book Top 100 Drug Interactions by Hansten & Horn)
2. Moxifloxacin is best against anaerobes but has no pseudomonas coverage
3. Moxifloxacin is hepatically metabolized – this is a double-edged sword as it has no dose adjustment in renal failure but can’t be used for a UTI

Example 5:

The Rule:

Bacteriocidal antibiotics work on the cell wall or DNA, bacteriostatic antibiotics work in the ribosomes

You need a ‘cillin to do the killin’.

The Exceptions:

1. Aminoglycosides work in the ribosome but are bacteriocidal
2. Linezolid is bacteriostatic against enterococci and staphylococci, and bactericidal for the majority of streptococci

Additional Resources

Although memorizing the rules and exceptions is very helpful, I still find myself using a tertiary reference to make sure I’m on the right track when it comes to antibiotic recommendations.  In addition to selecting empiric coverage, an antibiotic-specific reference helps with choosing the duration of therapy, additional tests that might be indicated, and provides alternatives in the face of multiple allergies.

John’s Hopkins Antibiotic Guide is my preferred antibiotic-specific reference.  I use the app version of this service, which needs to be renewed each year for about $30 to get continued updates.  I prefer it over other antibiotic resources for its clear and well thought out monographs, information on duration of therapy, and clinical pearls at the end of each topic.

Another resource is my free visual critical care antibiotic guide which I’ve updated for 2019.  The guide is a color-coded venn diagram that visually displays the coverage of most common antibiotics used in the ICU.  To download this guide to help you easily remember antibiotic spectrum of activity, go to pharmacyjoe.com/abx.

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If you like this post, check out my book – A Pharmacist’s Guide to Inpatient Medical Emergencies: How to respond to code blue, rapid response calls, and other medical emergencies.

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