Vancomycin-resistant Staphylococcus aureus laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Staph bacteria are classified as VISA or VRSA based on laboratory tests. Laboratories perform tests to determine if staph bacteria are resistant to antimicrobial agents that might be used for treatment of infections. For vancomycin and other antimicrobial agents, laboratories determine how much of the agent it requires to inhibit the growth of the organism in a test tube. The result of the test is usually expressed as a minimum inhibitory concentration (MIC) or the minimum amount of antimicrobial agent that inhibits bacterial growth in the test tube. Therefore, staph bacteria are classified as VISA if the MIC for vancomycin is 4-8µg/ml, and classified as VRSA if the vancomycin MIC is >16µg/ml.

Laboratory Findings

Algorithm for Testing S. aureus with Vancomycin (VA)

Copyleft image obtained courtesy of http://www.cdc.gov/HAI/organisms/visa_vrsa/visa_vrsa.html#a4

All laboratories should develop a step-by-step problem-solving procedure or algorithm for detecting VISA/VRSA that is specific for their laboratory. A sample algorithm is available here and highlights the recommended testing methodologies for detecting VISA/VRSA and actions based on results.

Not all susceptibility testing methods detect VISA and VRSA isolates. Three out of six confirmed VRSA isolates were not reliably detected by automated testing systems in a recent report. Subsequently, some manufacturers have optimized their systems for VRSA detection, so laboratories should check with manufacturers to determine if their system has FDA clearance for VRSA detection. VRSA are detected by reference broth microdilution, agar dilution, Etest®, MicroScan® overnight and Synergies plus™; BD Phoenix™ system, disk diffusion, and the vancomycin screen agar plate (brain heart infusion (BHI) agar containing 6 µg/ml of vancomycin).

VISA isolates are not detected by disk diffusion. Methods that typically detect VISA are non-automated MIC methods including reference broth microdilution, agar dilution, and Etest® using a 0.5 McFarland standard to prepare inoculum. Automated methods and vancomycin screen agar plates usually detect VISA for which the vancomycin MICs are 8 µg/ml, but further studies are need to define the level of sensitivity of these methods for S. aureus for which the vancomycin MICs are 4 µg/ml.