Updating antimicrobial susceptibility testing: methods.

1. Cynthia B. Schofield, MPH, MT (CAMT)[⇑][1] 1. VA San Diego Healthcare System (retired), San Diego, CA 1. Address for Correspondence: Cynthia B. Schofield, MPH, MT (CAMT), VA San Diego Healthcare System (retired), 7050 Weller St., San Diego, CA, (858) 450-9651, cschofield{at}san.rr.com 1. Explain the physician's choice of antibiotic agents according to CLSI guidelines. 2. Define and discuss MIC breakpoints. 3. Compare qualitative versus quantitative antimicrobial susceptibility testing (AST) methods. 4. Contrast the various quantitative AST methods. 5. Discuss the broth dilution reference method. 6. Discuss genotypic methods and their purposes. The importance of antimicrobial susceptibility testing (AST) as a clinical laboratory function escalates as organism resistance to the available antimicrobial agents increases. As patient outcome is based on antimicrobial therapy, standardization of the AST methods determining therapy is essential. The Clinical and Laboratory Standards Institute (CLSI) in the US and the European Committee on Antimicrobial Testing (EUCAST) in Europe determine these standards that are updated annually. Of concern to physicians are questions regarding the establishment of breakpoints, which are the interpretive cut-off values of minimal inhibitory concentration (MIC) in quantitative testing and inhibition zones for antimicrobial agents in qualitative testing based on geographical surveys of clinical isolates. The CLSI performance standards in the US and the EUCAST in Europe, the groups responsible for developing the breakpoints, may differ in perception and usage.1,2 Interpretation of qualitative results as susceptible (S), intermediate (I) or resistant (R) and quantitative measures (MICs) of antimicrobial activity are standardized by CLSI in the U.S. with breakpoint guidelines updated annually for each antibiotic tested.1,2,4 Antimicrobial Agents The clinical laboratory's list of antimicrobial agents is chosen by the infectious disease specialists and pharmacologists at the individual institution according to the institution's unique guidelines. A formulary of agents that physicians regularly prescribe is derived from the susceptibility testing of organisms typically isolated at each clinical laboratory and is guided by a monthly updated antibiogram. The number of agents tested is designated by the AST method used, e.g. 12 disks per 150 mm Mueller-Hinton agar plate or similar number per… ABBREVIATIONS: AST-Antimicrobial Susceptibility Testing; CLSI-Clinical and Laboratory Standards Institute; EUCAST-European Committee on Antimicrobial Susceptibility Testing; FDA-Food and Drug Administration; MIC-minimal inhibitory concentration; MH-Mueller-Hinton; CAMHB-cation-adjusted Mueller Hinton broth; ATCC-American Type Culture Collection; MRSA-methicillin-resistant Staphylococcus aureus ; VRE-vancomycin-resistant Enterococcus ; S, I, R-sensitive, intermediate, resistant; PCR-polymerase chain reaction; DNA-deoxyribonucleic acid; RT-reverse transcriptase; CAI-community-associated infection; HAI-hospital-associated infection; MSSA-methicillin-sensitive Staphylococcus sp.; PBP-penicillin-binding proteins; ESBL-extended-spectrum beta lactamase; KPC- Klebsiella pneumoniae , carbapenem-resistant; CFU-colony forming units; TSB-trypticase soy broth; CSF-cerebrospinal fluid; ESBL-extended-spectrum beta lactamase 1. Explain the physician's choice of antibiotic agents according to CLSI guidelines. 2. Define and discuss MIC breakpoints. 3. Compare qualitative versus quantitative antimicrobial susceptibility testing (AST) methods. 4. Contrast the various quantitative AST methods. 5. Discuss the broth dilution reference method. 6. Discuss genotypic methods and their purposes. [1]: #corresp-1