Monday, December 10, 2012

Beware the Antibiogram


As I was preparing a post discussing antimicrobial double coverage, I was describing the utility of a hospital antibiogram in aiding in the selection empiric therapy.  Let’s just say I’m glad I have had teachers and mentors whose favorite question was “why.”  While trying to answer that simple question, I came across this article from 2007 [1] that did an excellent job describing the limitations of antibiograms. Although I was aware of certain limitations of antibiograms, I wasn’t aware they are essentially useless in the ED.

Generally, antibiograms are annually reported collections of antimicrobial susceptibilities of local pathogens for a given institution.  Guidance on the standardization and construction of antibiograms is provided by CLSI (things like not reporting duplicate isolates, not reporting data for fewer than 10 isolates of a single bacterial species, providing subgroup analyses of patient populations, etc). But despite these published guidelines, data outlined in this review describe few hospitals are actually following these recommendations. 

Unfortunately, this translates into most antibiograms being an aggregate of all pathogens isolated by the microbiology lab in a given year. As a result, several limitations to the broad utilization of the antibiogram emerge (particularly its usefulness as a guide to empiric antimicrobial selection).

Antibiograms do not take into consideration a patient’s infection history, past antimicrobial use and comorbidities. For instance, for Pseudomonas specifically, antibiograms do not describe differing susceptibility patters with FQs and AGs in patients with and without cystic fibrosis.  Likewise, it cannot be determined whether the Pseudomonas isolate was from lung, urine or skin.

Since the collection of pathogen susceptibilities is often reported as aggregate, specific pathogen data should not be generalized across an entire hospital and between different patient care units.   Trends in specific hospital units tend to be hidden (Acinetobacter outbreak in the ICU).

Data reported in antibiograms does not provide information concerning the timing of the isolate in relation to infection (community vs hospital acquired).  Similarly, it cannot be deciphered whether the pathogen reported in the antibiogram was causing infection or colonization nor can they reveal trends in cross-resistance to multiple antimicrobials. This may be the biggest limitation preventing utilization of this tool in the ED.  It’s great to know my institutions’ antibiogram reports 72% susceptibility of levofloxacin to E. coli, but should that prevent prescribing an effective short course of antibiotics? It wouldn’t make sense to lump this suspected E.coli stain in with the one that was isolated from the chronically catheterized 85 year old patient who’s on day ICU day 10. In the antibiogram, it is.

What’s most interesting to me as a pharmacist is that these reports represent an aggregate proportion of susceptibility data for a given isolate-antimicrobial combination.  Similar to the above example, consider a 20-something year old UTI.  If the E.coli is likely to be resistant to levofloxacin, the antibiogram cannot help predict what this antimicrobial will be appropriate. Sure the susceptibility to nitrofurantoin is 99%, but does that change when the isolate is resistant to levofloxacin? No idea.

Antibiograms may still have a place when tailoring therapy for nosocomial infections. Certainly, if other institutions provide more detailed information on antibiograms, follow the standardization of construction recommendations or antibiograms evolve with new technology, their use could help guide empiric antimicrobial therapy. But in the ED, the current limitations prevent the broad utilization of antibiograms.

Pakyz AL. The utility of hospital antibiograms as tools for guiding empiric therapy and tracking resistance. Insights from the society of infectious disease pharmacists. Pharmacotherapy 2007;27(9):1306-12 

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