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Antimicrobial Susceptibility Testing

The primary role of the clinical microbiology laboratory is to provide information with which physicians can diagnose and treat infectious disease. The most important issues are whether an infectious agent is present and which antimicrobial agents will provide adequate therapy.

Mechanisms of bacterial resistance are complex and not completely understood. Likewise, antimicrobial susceptibility testing has become more challenging with the continued emergence of unique resistance mechanisms. The goal of the microbiology laboratory in antibiotic susceptibility testing is to provide standardized in vitro susceptibility tests that can be reproduced from day to day and from laboratory to laboratory. Without reproducibility there is no scientific basis for therapy. Standardized guidelines for susceptibility testing are published and updated annually by the Clinical Laboratory Standards Institute (CLSI). These guidelines provide susceptibility testing methods that have been validated as accurate, reproducible, clinically relevant and predictive of clinical efficacy based on pharmacokinetic and outcome data. Regulatory agencies, e.g. CLIA and CAP, expect microbiology laboratories to comply with CLSI guidelines for susceptibility testing.

Susceptibility testing of a presumed pathogen is indicated when its response to antimicrobial agents is not predictable. However, not all microbial pathogens and antimicrobial agents have been studied and validated by CLSI, and hence, the Microbiology laboratory does not routinely provide susceptibility results for these organisms. The most common reasons that susceptibility tests are not performed include:

  1. Antimicrobial-organism combination does not require testing because all strains are known to be either susceptible or resistant (e.g. group A strep vs. penicillin).
  2. Pathogen is so rarely recovered that too few clinical studies exist to establish testing standards (e.g. Vibrio species).
  3. Antimicrobial-organism combination is found to give erroneous and misleading susceptibility results (e.g. Enterococcus vs. cephalosporins).
  4. Organism does not grow well enough on standardized susceptibility media for testing to be performed.
  5. Drug-organism combination does not have adequate clinical response data to define MIC breakpoints (e.g. Corynebacteria and Bacillus species).
  6. Organisms recovered from culture represent normal human flora from the site of collection, or mixed flora from contamination of the collection site (e.g. skin flora from a wound swab or multiple gram-negative rods from a urine culture). Susceptibility testing is not indicated for normal flora or contaminating bacteria that are not responsible for the infection.

The majority of susceptibility testing is done by microbroth dilution. Serial dilutions of each antibiotic are inoculated with a standardized suspension of the bacteria being tested, then monitored for growth. The minimum inhibitory concentration (MIC) for a particular bacteria/ antibiotic combination is defined as the lowest concentration of antimicrobial agent in micrograms per milliliter that prevents the in vitro growth of bacteria.

MIC data is reported along with an interpretation of S, I, or R. In addition to the actual MIC number, other information that is critical in choosing an appropriate antibiotic includes half-life and achievable concentration at the site of infection. The physician should keep in mind that the antibiotic with the lowest MIC is not always the most appropriate choice of therapy.

The most useful means for assessing the adequacy of antimicrobial treatment in many infections is the clinical response of the patient to treatment and, if needed, demonstration by repeated culture that the infecting organism either has been eliminated or still persists. Antibiotic susceptibility tests are intended to be a guide for the clinician, not a guarantee that an antimicrobial agent will be effective in treatment, as many other in vivo factors may alter a patient’s response to therapy. Consultation with an infectious disease physician is recommended for assistance with complex antimicrobial therapy.

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