Antimicrobial Susceptibility Testing Pdf
Background: Antimicrobial susceptibility testing (AST) results are crucial for timely administration of effective antimicrobial treatment, and, thus, should be made available to clinicians as fast as possible. In particular, increasing rates of multidrug-resistant organisms emphasize the need for rapid AST (rAST). Manual for the Laboratory Identification and Antimicrobial Susceptibility Testing of Bacterial Pathogens of Public Health Importance in the Developing World. Haemophilus influenzae,Neisseria meningitidis, Streptococcus pneumoniae, Neisseria gonorrhoeae,Salmonella serotype Typhi. A number of antimicrobial susceptibility testing (AST) methods are available to determine bacterial susceptibility to antimicrobials. The selection of a method is based on many factors such as practicality, flexibility, automation. The antimicrobial agents used in medical practice are aimed at eliminating the infecting microorganisms or at preventing the establishment of an infection. To be of therapeutic use, an antimicrobial agent must exhibit selective toxicity; it must exhibit greater toxicity to the infecting pathogens than to the host organism.
Performance Standards for Antimicrobial Susceptibility Testing, 28th Edition. This document includes updated tables for the Clinical and Laboratory Standards Institute antimicrobial susceptibility testing standards M02, M07, and M11. The tables in M100 are intended for. On Antimicrobial Susceptibility Testing of bacteria and fungi This brochure is intended to provide succinct answers to common questions about the performance of in vitro antimicrobial susceptibility testing of bacteria and fungi and the value of the results in guiding antimicrobial therapy. Antimicrobial susceptibility testing of H. Influenzae This laboratory manual describes susceptibility testing of H. Influenzae by the disk diffusion method and by the antibiotic gradient strip testing method. Although disk diffusion will provide information as to whether a strain is susceptible, intermediate, or resistant, the gradient strip method provides more detailed information about the minimal inhibitory concentration (MIC) of an antimicrobial agent.
INTRODUCTION
There are few activities in the clinical microbiology laboratory that utilize more technologist time and laboratory resources than antimicrobial susceptibility testing (AST). It has been suggested that, at least in terms of direct relevance to the care of patients with infection, AST may be the single most important activity performed in the clinical microbiology laboratory. AST results are often used to dictate specific management for individual patients, summary AST data is used to drive empiric antimicrobial therapy, and, finally, formulary decisions in some cases are made based on AST results from the laboratory. Thus, questions arise. How well does the clinical microbiology laboratory perform when providing AST services? And are there specific areas in which improvements can be achieved? With the aim of addressing these questions, four different topics were identified for discussion.
The first topic of discussion was the manner in which interpretive criteria for MIC results, i.e., breakpoints, are developed. Breakpoints provide the basis for categorizing the results of in vitro susceptibility tests into predictions of outcome. Generally, three MIC categories are defined: susceptible (S), intermediate (I), and resistant (R). These terms, when provided in susceptibility test reports to clinicians charged with caring for patients with infection, clearly drive therapeutic decision-making. How is it that a quantitative in vitro measure of antibiotic effect, i.e., an MIC, can be converted into a predictor of therapeutic effect?
As with most things these days, it happens by committee. The Clinical and Laboratory Standards Institute (CLSI; formerly the National Committee for Clinical Laboratory Standards, or NCCLS) in the United States and the European Union Committee for Antimicrobial Susceptibility Testing (EUCAST) in Europe have assumed the responsibility for developing and promulgating MIC breakpoints. These two organizations employ some combination of four criteria when developing MIC breakpoints: MIC frequency distribution analysis, assessment of MIC values in the context of the presence or absence of known mechanisms of resistance, evaluation of MICs based on drug levels in patients receiving antibiotic therapy, i.e., pharmacokinetic and pharmacodynamic (PK/PD) analysis, and, finally, clinical correlation (that is, response rates in patients with infection compared to the drug MICs associated with their infecting pathogens). Each of these approaches has both strengths and weaknesses as a means for developing MIC breakpoints, and those strengths and weaknesses serve as the focus of the first discussion in this section.
As an aside, the CLSI has traditionally been the international leader in the business of establishing MIC breakpoints; however, largely because of the more expeditious, reasoned, and consistent approach of EUCAST, the methods of the European entity have more recently begun to gain traction internationally. While there have been some efforts to work together, currently CLSI and EUCAST remain separate organizations, in many cases promulgating different breakpoints for the same drugs and bugs. This, of course, makes no sense unless the different breakpoints are specifically based on predicting outcome with different drug-dosing algorithms or are predicated on different testing methods. Clearly, the CLSI and EUCAST must coalesce into a single organization. It is the hope of the organizers of Camp Clin Micro that CSLI and EUCAST will join together in an effort to develop a single international standard for in vitro susceptibility testing.
The second discussion in this section concerns the actual clinical predictive value of in vitro susceptibility testing. Let us assume that a laboratory generates a susceptibility test result using a reference standard method, interprets the result using the defined breakpoint criteria of either the CLSI or EUCAST, and then reports that result to a clinician. What is the likelihood of that result reliably predicting either a favorable or unfavorable therapeutic response? In other words, what is the actual clinical predictive value of in vitro susceptibility testing? By any reasonable measure, the clinical predictive value defines the accuracy of susceptibility testing. Unfortunately, as reviewed in the second discussion of this section, current AST methods fall woefully short of accurately predicting outcomes for many patients with infection. Possible solutions to this problem are presented.
In the third discussion of this section, numerous issues surrounding the reporting of AST results are considered. The importance of testing appropriate antibiotics is emphasized, and insight is provided regarding how best to select specific drugs for testing. The use of institution-, care area-, or patient-specific algorithms for reporting AST results is discussed, and guidance is provided as to the best way to accomplish this. The importance of direct personal interaction with care providers as a means of optimizing the use of AST results is outlined. In addition, the construction and distribution of cumulative summaries of AST results are discussed. Finally, several novel approaches to reporting AST results are presented.
In the last discussion of this section, issues surrounding the direct molecular detection of resistance determinants either from organisms recovered regarded as representative of infection in the laboratory or directly from clinical material are considered. The use of molecular methods to define the presence or absence of resistance determinants may represent an alternative to traditional phenotypic susceptibility testing, allowing researchers to, as it were, skip “Go” and directly look for factors known to account for resistance among pathogenic bacteria. The principal appeal of such an approach is that it focuses on the factors that account for therapeutic failure and thus obviates some of the vagaries associated with attempts to make clinical sense out of MICs and their interpretation using breakpoints. The strengths and weaknesses of direct detection of resistance determinants are discussed, and several specific examples of where this novel approach to susceptibility testing currently has clear application are elucidated.
This course is part of the Methods of Antimicrobial Susceptibility Testing Educational Resource (MASTER) eLearning Series.
Description
This course provides a detailed overview of antimicrobial susceptibility testing methods including reference methods such as those described by the Clinical and Laboratory Standards Institute (CLSI) and commercial test systems. It focuses on technical and regulatory considerations for any method of medical laboratory antimicrobial susceptibility testing. The course provides essential information for those new to antimicrobial susceptibility testing and an important refresher for laboratory professionals with more experience.
Objectives
At the conclusion of this eLearning, you will be able to:
- Identify the steps required to perform the Clinical and Laboratory Standards Institute (CLSI) disk diffusion and Minimal Inhibitory Concentration (MIC) reference methods
- Identify the key steps that must be modified when performing disk diffusion or MIC procedures for fastidious organisms
- Review the importance and limitations of commercial antimicrobial susceptibility test systems
Continuing Education Units (CEU)
The Centers for Disease Control and Prevention, Division of Laboratory Systems, Laboratory Training and Services Branch, is approved as a provider of continuing education programs in the clinical laboratory sciences by the ASCLS P.A.C.E.® Program. This course is approved for 1.5 contact hours of P.A.C.E. ® credit.
P.A.C.E.® Course Number: 288-023-17
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This course has been approved for 1.5 contact hours in the category of Microbiology/Mycology/Parasitology for Florida Laboratory Licensees.
Florida Course Number: 20-434651
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