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  • Rapid preliminary identification by staining, microscopic evaluation (Table 16-1).

  • Antibiotic therapy may be initiated or broadened based on Gram stain interpretation. Therapy should NOT be narrowed based on a Gram stain.

TABLE 16-1

Gram Stain Evaluation


  • MIC: minimum inhibitory concentration, or the lowest concentration of an antibiotic that inhibits the growth of a particular bacteria (Figure 16-1).

  • Breakpoint: the cutoff that defines susceptibility for an antibiotic and organism pair

    • Breakpoints are determined and updated by the Clinical and Laboratory Standards Institute.1

    • Breakpoint cutoffs take into consideration MIC distributions, pharmacokinetics, pharmacodynamics, mechanisms of resistance, and clinical outcome data assessing safety and efficacy.

    • MICs should not be compared across different antibiotics in a susceptibility panel.

    • Antibiotic MICs should only be compared to the breakpoint of the same antibiotic.

    • MIC values are not comparable between different antibiotics. For example, if antibiotic A has a lower MIC compared to antibiotic B, that does NOT necessarily indicate antibiotic A is a better choice.

    • The lower a MIC is from the breakpoint of susceptibility, the more likely that antibiotic will be able to achieve the necessary concentration at the site of action.

    • The intermediate susceptibly category serves as a buffer between the susceptible and resistant categories. Infections due to intermediate susceptible organisms may be treatable if higher doses are used or if the antibacterial is concentrated at the site of infection.

  • Limitations and Pearls

    • The choice of antibiotic should be based on the MIC, the breakpoint, and the site of infection.

    • Some MIC breakpoints may differ depending on the site of infection. For example, the breakpoint for ceftriaxone susceptibility when treating Streptococcus pneumoniae in the bloodstream is ≤ 1, whereas the breakpoint for treating the same organism in the meninges is 0.5.

    • Many antibiotics concentrate in the urine and may be used to treat urinary tract infections (UTIs) even when nonsusceptible by in vitro testing.

    • A cephalosporin or penicillin derivative should not be used to treat a serious infection caused by an ESBL (extended-spectrum beta-lactamase)–producing gram-negative bacterium even if considered “sensitive” to cephalosporins per MIC breakpoint.

    • Isolates initially susceptible may become intermediate or resistant after initiation of therapy. This is most frequently seen when a third-generation cephalosporin is used to treat serious Enterobacter, Citrobacter, or Serratia infections, as a result of inducible amp-C beta-lactamases.

    • Interpretation ...

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