When it comes to memorizing spectrum of activity for antibiotics, I first learn the rules for a class of antibiotics, then I focus on learning the exceptions to the rules.
Here are 4 examples:
Rule #1 MRSA is covered by vancomycin, linezolid, daptomycin, and ceftaroline.
Exceptions:
- Daptomycin is inactivated by lung surfactant – don’t use in pneumonia
- Linezolid is bacteriostatic – don’t use in bloodstream infections (although it can be used for VRE bacteremia)
- Ceftaroline is vancomycin and ceftriaxone rolled into one loveable fuzzball
Rule #2 Piperacillin-tazobactam and the carbapenems ertapenem, meropenem, doripenem, and imipenem cover gram positives, gram negatives, and anaerobes.
Exceptions:
- Ertapenem doesn’t cover pseudomonas
- Piperacillin-tazobactam doesn’t cover ESBL well enough to be used in severe infections (although it probably works for UTI)
Rule #3 Cephalosporins for empiric use in the ICU are cefepime, ceftriaxone, ceftazidime, and ceftaroline. They cover gram positives and gram negatives.
Exceptions:
- Only cefepime and ceftazidime for pseudomonas
- None cover enterococcus (remember – this shows up on the gram stain as “gram-positive cocci resembling strep!”)
Rule #4 Quinolones ciprofloxacin, levofloxacin, and moxifloxacin cover gram positives and gram negatives (double check your local resistance patterns!!!).
Exceptions:
- Ciprofloxacin is weak against strep pneumoniae, 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)
- Moxifloxacin is best against anaerobes but has no pseudomonas coverage
- 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
Members of my Hospital Pharmacy Academy have access to the ICU Drugs & Bugs Masterclass, which provides in-depth explanations of the spectrum of activity and empiric antibiotic selection for critically ill patients. Click here to learn more and join the Academy.