When should I pay attention to the minimum inhibitory concentration (MIC) of an antibiotic despite the lab reporting it to be in the “Susceptible” range?

For most clinicians, the choice of antibiotic is based on the laboratory reporting of “Susceptible” (vs “Resistant”), not the actual MIC value of the drug.  However, despite the fact that much of the data is retrospective and the association of MIC within a susceptible range and clinical outcome may not necessarily be causal, many experts recommend caution when “high” MICs within a susceptible range are observed, as found in the following situations:   

  1. Vancomycin MIC >1 ug/ml in Staphylococcal aureus (methicillin-sensitive or –resistant) infections which may be associated with clinical failure and, at times, increased mortality1,2.
  2. Ciprofloxacin or levofloxacin MIC>0.25 ug/ml in bacteremia caused by Gram-negative bacilli (including Enterobacteriacae as well as Pseudomonas aeruginosa) which may be associated with adverse outcome (eg, longer average hospital stay post-culture and duration of infection) but not necessarily mortality3-5.
  3. Levofloxacin MIC ≥ 1.0 ug/ml in Streptococcus pneumoniae infections, which may be associated with adverse clinical outcome based on drug pharmacodynamics and anecdotal reports of treatment failure6,7.

 

Contributed in part by Nick Van Hise, Pharm.D., BCPS, Infectious Diseases Clinical Pharmacist, Edward-Elmhurst Hospitals, Naperville, Illinois

References

  1. Jacob JT, DiazGranados CA. High vancomycin minimum inhibitory concentration and clinical outomces in adults with methicillin-resistant Staphylococcus aureus infections: a meta-analysis. Int J Infect Dis 2013;17:e93-e100.
  2. Kalil AC, Van Schooneveld TC, Fey PD, et al. Association between vancomycin minimum inhibitory concentration and mortality among patients with Staphylococcus aureus bloodstream infections: A systematic review and meta-analysis. JAMA 2014;312:1552-1564.
  3. DeFife R, Scheetz MH, Feinglass J, et al. Effect of differences in MIC values on clinical outcomes in patients with bloodstream infections caused by Gram-negative organisms treated with levofloxacin. Antimicrob Agents Chemother 2009;53:1074-79.
  4. Falagas ME, Tansarli GS, Rafailidis PI, et al. Impact of antibiotic MIC on infection outcome in patients with susceptible Gram-negative bacteria a systematic review and meta-analysis. Antimicrob Agents Chemother 2012;56:4214-22.
  5. Zelenitsky SA, Harding GKM, Sun S, et al. Treatment and outcome of Pseudomonas aeruginosa bacteremia: an antibiotic pharmacodynamics analysis. J Antimicrob Chemother 2003;52:668-674.
  6. Davidson R, Cavalcanti R, Brunton JL, et al. Resistance to levofloxacin and failure of treatment of pneumococcal pneumonia. N Engl J Med 2002;346:. 2002;346:747-50.
  7. De Cueto M, Rodriguez JM, Soriano MJ, et al. Fatal levofloxacin failure in treatment of a bacteremic patient infected with Streptococcus pneumoniae with a preexisting parC mutation. J Clin Microbiol 2008;46:1558-1560.
When should I pay attention to the minimum inhibitory concentration (MIC) of an antibiotic despite the lab reporting it to be in the “Susceptible” range?

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