Updating Antimicrobial Susceptibility Testing Introduction

Abstract

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. Identify the challenges of reporting and interpreting antimicrobial susceptibility testing results. 2. Describe the current methods utilized to determine AST results. 3. Discuss the correlation of AST results with patient outcome. Controversy exists concerning breakpoints or interpretive criteria, which are the values that determine the antimicrobial susceptibility testing (AST) categories susceptible, intermediate and resistant, the clinical predictive value of the minimal inhibitory concentration (MIC) criteria and the method of reporting AST results.1,2,3 Agreement between the physician and the clinical laboratory regarding the laboratory's interpretation, prediction and reporting of AST results is a potential source of conflict. While the physician's primary concern relates to the therapeutic dosing and the patient's clinical outcome, the latter is bound by the often inflexible type of standardization promoted by the Clinical and Laboratory Standards Institute (CLSI).1,3 Methods of testing, rigidity of quality control and adherence to the annually updated guidelines of the CLSI are paramount in the United States. A different concept is used by the European Committee on Antimicrobial Susceptibility Testing (EUCAST). This system uses separate values and clinical breakpoints to note a microorganism as wild type (WT) or non-wild type (NWT). An organism lacking mechanisms of acquired or mutational resistance is used to define WT. The presence of one or more mechanisms of resistance defines NWT when an antibiotic is considered. The purpose of the EUCAST system is to detect small and large changes in susceptibility that will reflect the constantly evolving mechanisms of antimicrobial resistance. Variation in breakpoints from one system to another contributes to the complexity of interpretation and obscurity of prediction value most noted by physicians.1 In cases of acute infection, patients are treated empirically as bacterial identification and susceptibility reporting… ABBREVIATIONS: MIC-minimal inhibitory concentration; AST-antimicrobial susceptibility testing; CLSI-Clinical and Laboratory Standards Institute; EUCAST-European Committee on Antimicrobial Susceptibility Testing; WT-wild type; NWT-non-wild type; UTI-urinary tract infection; MDR-multi-drug resistant; ESBL-extended-spectrum beta lactamase 1. Identify the challenges of reporting and interpreting antimicrobial susceptibility testing results. 2. Describe the current methods utilized to determine AST results. 3. Discuss the correlation of AST results with patient outcome. [1]: #corresp-1