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Quality Control Issues in Antibiotic Susceptibility Testing by Disc Diffusion Technique

Abstract

Vivek Bhat and Hemant Vira

Although automated susceptibility methods are widely available nowadays, antimicrobial susceptibility testing performed by the Kirby Bauer technique is still being commonly used in many laboratories especially in the developing countries. They provide a simple and inexpensive method for determining susceptibility of various Gram positive and Gram negative organisms against various drugs. The quality of results generated by the laboratory with respect to susceptibility of microorganisms to antimicrobials has an important bearing on patient management and outcomes. Any quality assurance program must include internal quality control (IQC) and external quality assessment. The important IQC measures in disc diffusion susceptibility testing include use of specific Quality Control (QC) strains, batch testing of media and antibiotics, and lot-to-lot verifications of reagents used. QC may be performed daily or weekly as per the workload of the laboratory. Some important causes of QC failure include improper storage of QC strains, media and reagents; errors in inoculums preparation and incubation temperatures; and equipment related issues like calibration errors. Accuracy and reliability of results should be regularly assessed by proficiency testing. If any failure in the QC is noted, corrective and preventive actions must be undertaken to ensure the quality of results generated.