I. MIC Information
What is an MIC?
- A Minimum Inhibitory Concentration (MIC) is the lowest concentration of an antibacterial agent expressed in µg/mL which, under strictly controlled in vitro conditions, completely prevents visible growth of the test strain of an organism
Why do MICs matter to me?
- Adequate drug exposure is critical to ensuring positive patient outcomes
- Drug exposure is directly impacted by the MIC
- As the MIC rises, drug exposure decreases
- Antibiotic resistance can be induced from inadequate drug exposure
- Appropriate dosing of antimicrobials is key!
How are MICs determined?
- Most of the MIC results we see in Epic come from automated testing methods
- Some bug/drug combinations require manual MIC testing methods (Table 1)
Note: MIC variability can occur between susceptibility techniques
How do I interpret MIC breakpoints?
How are breakpoints set?
- Antimicrobial susceptibility standards are currently set by the FDA. The FDA typically refers to standards set by the Clinical and Laboratory Standards Institute (CLSI).
- The CLSI uses microbiologic data, pharmacokinetic and pharmacodynamic data, and clinical studies to establish breakpoints.
- Breakpoints may change over time as additional information becomes available.
- The Clinical and Laboratory Standards Institute M100 is available online as a free resource and offers the most up-to-date information on breakpoints.
Questions? Contact ASET!
References:
- EUCAST Definitive Document. Methods for the determination of susceptibility of bacteria to antimicrobial agents. Terminology. Clin Microbiol Infect 1998;4:291–296.
- Kowalska-Krochmal B, et al. The Minimum Inhibitory Concentration of Antibiotics: Methods, Interpretation, Clinical Relevance. Pathogens 2021;10:165. doi:10.3390/ pathogens10020165.
- Giuliano C, et al. A Guide to Bacterial Culture Identification and Results Interpretation. P T 2019;44(4):192-200.
- Reller LB, et al. Antimicrobial Susceptibility Testing: A Review of General Principles and Contemporary Practices. Clinical Infectious Diseases 2009;49(11):1749–1755. https://doi.org/10.1086/647952.
- The United States Committee on Antimicrobial Susceptibility Testing (USCAST). USCAST - Home. Accessed February 26, 2024.
- The Clinical and Laboratory Standards Institute. CLSI eClipse Ultimate Access - Powered by Edaptive Technologies (edaptivedocs.net). Accessed February 26, 2024.
- American Society for Microbiology. Updating Breakpoints in Antimicrobial Susceptibility Testing. https://asm.org/articles/2022/february/updating-breakpoints-in-antimicrobial-susceptibili. Updated December 13, 2023. Accessed February 26, 2024.
- Wenzler E, et al. Antimicrobial susceptibility testing: An updated primer for clinicians in the era of antimicrobial resistance: Insights from the Society of Infectious Diseases Pharmacists. Pharmacotherapy 2023;43:264-278. doi:10.1002/phar.2781.
- Holmes NE, Turnidge JD, Munckhof WJ, et al. Antibiotic choice may not explain poorer outcomes in patients with Staphylococcus aureus bacteremia and high vancomycin minimum inhibitory concentrations. J Infect Dis. 2011;204(3):340-347.
II. Phoenix Susceptibility Testing Limitations (see attachment)
- Limitation: Serratia marcescens, TMP-SMX not on/reliable from the panel
- Solution: TMP-SMX will be set up simultaneously for all S. marcescens
- Limitation: Proteus mirabilis, meropenem not on/reliable from the panel
- Solution: If cefepime = SDD/R, set up/release meropenem Etest (results available next day)
- Limitation: Proteus vulgaris/penneri, ampicillin-sulbactam not on/reliable from the panel
- Solution: A comments to call the lab if this agent is clinically indicated will be added each time these organisms are isolated