Infectious Diseases II
prescribed doses will meet specified PD criteria for suppressing bacterial growth
Breakpoints that may decrease the need for confirmatory tests, hence decreasing the microbiology
laboratory workload. Such an approach, although possibly sensitive for determining the presence of
resistance mechanisms, may potentially lead to the compensatory use of broader antimicrobials.
The CLSI breakpoints may be inconsistent with the breakpoints established by the FDA. Automated
systems report MIC breakpoints according to FDA approvals. Changes to reporting and labeling
require additional clearance from the FDA. Unfortunately, the FDA labeling may not be up to date with
the current CLSI recommendations. Hence, despite newer CLSI recommendations, many institutions
may still be reporting susceptibility results that are based on their respective FDA labeling breakpoints.
These breakpoints may result in pathogens being labeled as sensitive, even though, according to the
current CLSI standards, they would be considered resistant.
The exact MIC breakpoints that are used by a local microbiology laboratory can be variable, depending
on the laboratoryβs adaptation of new standards. In addition, there are significant variations in the FDA-
approved breakpoints used by automated AST methods and updated CLSI guidelines. Once again, this
highlights the importance of a clinicianβs understanding of the exact methods and breakpoints that are
used in his or her institution.
microbiology laboratory practices
Routine methods for AST
If additional tests are available (E-test, genotyping, etc.) and how to go about requesting additional
information
Are confirmatory tests for resistance mechanisms routinely done (e.g., double-disk diffusion for
macrolide-lincosamide-streptograminβinducible resistance, ESBL and KPC confirmatory tests)?
What is the usual procedure for censoring the AST results?
Does the AST result try to guide clinical decision (i.e., if pathogen were sensitive to oxacillin,
would vancomycin results be censored)?
If certain resistance mechanisms are detected, would the laboratory automatically update
susceptibility results (i.e., if ESBL were detected, would Ξ²-lactam/Ξ²-lactamase combination
antibiotics automatically be changed to resistant)?
Are any antibiotics part of the standard panel but routinely hidden from clinical reports?
| d. | Most microbiology laboratories would retain all the information, even if parts of the results had |
|---|
been censored. Critical care pharmacists should consider contacting the microbiology laboratories
to see whether additional information regarding the pathogen is available.
When an MIC is reported, is only the MIC breakpoint reported (i.e., MIC 2 mcg/mL or less), or is the
actual MIC reported (i.e., 0.25 mcg/mL)?
Are the breakpoints reported consistent with the updated CLSI recommendations?
The ideal application of AST interpretation and clinical therapeutics may include the following:
Knowledge of the individual pathogen and the corresponding MIC breakpoints for each of the
tested antibiotics
Potential reasoning behind each of the clinical breakpoint designations (i.e., wild-type selection,
PK/PD breakpoints assessment, screening for resistance)
Accurate comprehension of the local AST methods used and potential limitations of the MIC
reported