Introduction to Antimicrobial Susceptibility Testing

Antimicrobial susceptibility testing, also known as antibiogram testing or antibiotic sensitivity testing, is a laboratory technique used to determine the sensitivity of bacteria, fungi, or other microbes to different antimicrobial drugs such as antibiotics, antifungals, or antivirals. This testing helps clinicians determine the most appropriate antibiotics for treatment of infections caused by different microorganisms. AST provides crucial data to guide empiric therapy and limit the development and spread of antibiotic resistance.

Importance of AST in Clinical Practice

Knowing the Antimicrobial Susceptibility Testing profile of the infecting pathogen helps optimize patient care and outcomes. AST guides clinicians in selecting the most appropriate antibiotic therapy based on which antimicrobial drugs are likely to be effective against that specific infection. This improves clinical cure rates and reduces exposure to broad-spectrum antibiotics that could drive resistance. Empiric therapy selected without guidance from AST may expose the patient to unnecessary costs and potential side effects from antibiotics that do not target the actual pathogen. Accurate AST also aids in hospital infection control by limiting spread of resistant bugs.

Common AST Methods

Several standard laboratory techniques are used for AST, with culture-based methods being the gold standard. Disk diffusion testing involves placing antibiotic-impregnated paper disks onto agar plates inoculated with the test organism. Inhibition zone diameters around each disk are measured after incubation to determine susceptibility. Broth microdilution uses plates with serial dilutions of antibiotics in wells inoculated with the organism; the minimum inhibitory concentration (MIC) yielding no growth is reported. Newer rapid methods like automated systems, molecular tests, and sequencing can detect resistance genes and provide results in hours versus days.

AST in the Era of Multidrug Resistance

Antimicrobial resistance, driven by inappropriate antibiotic use, is a growing public health crisis. Monitoring resistance trends through AST programs is critical for surveillance and response. AST guides infection control interventions to limit spread of resistant organisms. It also informs empiric prescribing guidelines and antibiotic stewardship policies at healthcare facilities. Molecular characterization through AST sheds light on Resistance mechanisms and how they emerge and disseminate between regions and patient populations. Advances in AST technology accelerate optimized care and development of novel antibiotics desperately needed to combat resistant superbugs.

Reporting and Interpreting AST Results

After performing phenotypic or genotypic testing, laboratories report AST results and interpret them per standardized guidelines. Breakpoints define classifications of susceptible, intermediate, or resistant based on an agent's potency against a specific pathogen. Lab reports include the identification of the infecting organism and its categorized susceptibility to various drugs. Clinicians then refer to these results along with clinical context to select the most suitable antibiotic and duration of therapy. As susceptibility profiles for facilities are also compiled, they render cumulative antibiograms useful for comparing local resistance trends and formulating empiric prescribing guidance.

Quality Assurance in AST

Robust quality assurance practices are crucial for AST as results directly guide clinical decision-making. Internal quality control includes testing reference strains of known susceptibility periodically to validate assay performance run-to-run and technician-to-technician. External proficiency testing programs provide blinded samples to labs to assess AST accuracy compared to peer performance. Participation in referral networks enables correlations with reference laboratories. Standardized methods, controls, training and certification of personnel uphold reliable and reproducible AST contributing to prudent antibiotic use. Continuous quality improvement refines practices and helps laboratories deliver clinically relevant and actionable AST results.

Antimicrobial susceptibility testing is a valuable tool that supports optimal antimicrobial stewardship and management of infectious diseases. It provides crucial information to effectively treat infections while slowing the development and spread of resistance. Advancing AST techniques along with quality standards ensures patients continue receiving timely, targeted and life-saving antibiotic therapy in the face of mounting antimicrobial resistance challenges.

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