Sanford Guide's Stewardship Alerts are educational resources developed by our Antimicrobial Stewardship Program Manager, Ann Lloyd, Pharm.D., BCPS, BCIDP. The content of our Stewardship Alerts may not reflect the views of the Sanford Guide editorial board and intended for educational use only. A printer friendly version of this resource is available for download here.
Antimicrobial Susceptibility Testing and Minimum Inhibitory Concentration
A reversal of progress toward reducing antibiotic resistant infections was seen during the COVID-19 pandemic. Antibiotic prescribing increased, and when coupled with challenges in infection prevention and control guidance, healthcare-associated, antimicrobial-resistant infections increased as well.1 To counteract these changes, antimicrobial stewardship (AMS) remains critical. AMS programs are designed to improve antibiotic use through selection of the right drug, at the right dose, using the right route, at the right time, and for the right duration of therapy.2
Annually, the Centers for Disease Control and Prevention observes U.S. Antibiotic Awareness Week (USAAW) to highlight the threat of antibiotic resistance and emphasize the importance of appropriate antibiotic use. This year, USAAW is November 18-24, 2023, and one component of the campaign is the 5 Ways Hospital Pharmacists Can be Antibiotics Aware.3 This includes verifying penicillin allergies, avoiding duplicative anaerobic coverage, reassessing antibiotic therapy, avoiding treatment of asymptomatic bacteriuria, and using the shortest effective duration of therapy. This Stewardship Alert will focus on reassessing antibiotic therapy through review of microbiology results and stopping or tailoring antibiotic therapy. To best accomplish this, it is important to understand antimicrobial susceptibility testing (AST) and the minimal inhibitory concentration (MIC).
What is Antimicrobial Susceptibility Testing?4-8
Antimicrobial susceptibility testing is a procedure conducted in a microbiology lab to identify which antibiotic is effective for an individual patient. AST is useful in predicting the outcome of treatment with the antimicrobial agents being tested. It is helpful for clinicians to determine susceptibility to the empiric agent and detect potential resistance. AST methods are continuously evolving, but historically have included qualitative and quantitative methods with most laboratories using automated systems to conduct the testing. Interpretation of AST uses clinical breakpoints to divide results into categories which correlate with the likelihood of clinical outcomes.
What is the minimal inhibitory concentration (MIC) and how is it used to interpret AST results?5,8-10
The MIC is the lowest concentration of an antimicrobial agent that prevents visible growth of an organism in an agar or broth dilution susceptibility test. The susceptibility breakpoint, or interpretative criteria, is the MIC value used to categorize susceptible, intermediate, and resistant categories. These categories are derived from microbiology characteristics, pharmacokinetic/pharmacodynamic parameters, and clinical outcomes data if available. A susceptible result suggests a high probability that the patient will respond to therapy using the appropriate dosage regimen while a resistant result indicates that treatment with the antimicrobial agent is likely to fail. An intermediate result includes isolates with MICs within the intermediate range that approach typically attainable blood and tissue levels and/or for which response rates may be lower than for susceptible isolates.
How are susceptibility breakpoints established?8,11
The three organizations involved with setting AST breakpoints include the Clinical and Laboratory Standards Institute (CLSI), the Food and Drug Administration (FDA), and the European Committee on Antimicrobial Susceptibility Testing (EUCAST) working with the United States Committee on Antimicrobial Susceptibility Testing (USCAST). While there is some harmonization of breakpoints across the organizations, several differences exist particularly around the use of the intermediate interpretative criteria.
Breakpoints are initially established during the drug development phase but often require revision over time due to new resistance mechanisms, pharmacokinetic/pharmacodynamic data, new dosing methods, etc.. In the United States, manufacturers of commercial AST platforms must use FDA breakpoints, so implementation of breakpoint changes can take time. Laboratories can adopt breakpoint changes through internal lab validation, but this process can take several weeks.
How can AMS programs use AST results to optimize antibiotic use?2,6-7
AMS programs can use AST results to reassess antibiotic therapy and recommend adjustments to provide optimal outcomes for the patient. Selecting the antibiotic with the lowest MIC on an AST panel does not mean that agent has the best chance of efficacy. Each organism-drug concentration varies, and interpretation of the MIC should include using information about the pharmacokinetics of the drug and the likelihood of the antibiotic in eliminating the bacteria at different body sites.
AMS programs and microbiology laboratories should work together to optimize the use of AST. Some may consider only reporting the interpretative criteria (susceptible, intermediate, and resistant) and releasing the MIC values only to infectious diseases experts. Another common intervention is selective or cascade reporting of AST results. This involves reporting AST results for limited antibiotics instead of all tested agents to increase the appropriateness of antibiotic prescribing. Other opportunities include working together to prioritize incorporation of CLSI breakpoint changes.
- Centers for Disease Control and Prevention. COVID-19: U.S. Impact on Antimicrobial Resistance, Special Report 2022. Atlanta, GA: U.S. Department of Health and Human Services, CDC; 2022. https://www.cdc.gov/drugresistance/covid19.html
- Barlam TF, Cosgrove SE, Abbo LM, et al. Implementing an antibiotic stewardship program: guidelines by the Infectious Diseases Society of American and the Society for Healthcare Epidemiology of America. Clin Infect Dis. 2016;62(10):e51-e77. https://doi.org/10.1093/cid/ciw118
- Centers for Disease Control and Prevention. 5 ways hospital pharmacists can be antibiotic aware. Available at: https://www.cdc.gov/antibiotic-use/community/pdfs/Hospital-Pharmacist-Poster-508.pdf Accessed August 22, 2022.
- Bayot ML, Bragg BN. Antimicrobial Susceptibility Testing. [Updated 2021 Oct 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539714/?report=classic
- Turnidge JD. Susceptibility test methods: general considerations. In: Jorgensen JH, et al., eds. Manual of Clinical Microbiology. Vol 1. 11th ed. ASM Press; 2015: 1246-52.
- Kuper KM, Boles DM, Mohr JF, et al. Antimicrobial susceptibility testing: a primer for clinicians. Pharmacotherapy. 2009;29(11):1326-1343.
- Jorgensen JH, Ferraro MJ. Antimicrobial susceptibility testing: a review of general principles and contemporary practices. Clin Infect Dis. 2009;49(11):1749-1755.
- Humphries RM, Abbott AN, Hindler JA. Understanding and addressing CLSI breakpoint revisions: a primer for clinical laboratories. J Clin Microbiol. 2019;57(6):e00203-19. doi:10.1128/JCM.00203-19
- Clinical and Laboratory Standards Institute. CLSI M23-ED5: 2018 development of in vitro susceptibility testing criteria and quality control parameters, 5th edition. Available at: http://em100.edaptivedocs.net/Login.aspx. Accessed August 22, 2022.
- Clinical and Laboratory Standards Institute. CLSI M100-ED32: 2022 performance standards for antimicrobial susceptibility testing, 32nd edition. Available at: http://em100.edaptivedocs.net/Login.aspx. Accessed August 22, 2022.
- Weinstein MP, Lewis JS. The Clinical and Laboratory Standards Institute subcommittee on antimicrobial susceptibility testing: background, organization, functions, and processes. J Clin Microbiol. 2020;58(3):e01864-19. doi:10.1128/JCM.01864-19