[Table of Contents]

Canada Communicable Disease Report - Supplement
Volume: 23S8
December 1997

INFECTION CONTROL GUIDELINES

Preventing the Spread of Vancomycin-Resistant Enterococci (VRE) in Canada

Antimicrobial Susceptibility Testing of Enterococci

Antimicrobial susceptibility testing can be performed using different methods. The traditional methods have been broth or agar dilution and disk diffusion. More recently, automated systems have gained wide usage for antimicrobial susceptibility testing. Accurate test results are crucial for both patient management and infection control measures. All the methods mentioned reliably detect strains exhibiting the higher levels of vancomycin resistance (MIC > 128 µg/mL). However, disk and automated systems have varied in their abilities to detect low to moderate levels of resistance (8-64 µg/mL)^(50-52). The National Committee for Clinical Laboratory Standards (NCCLS) has made recommendations on the procedures and interpretive criteria used in disk diffusion testing for vancomycin and teicoplanin, which should be followed to ensure reliable detection of VRE⁽⁵³⁾. Willey et al. found an agar screening plate with brain heart infusion (BHI) agar incorporating 6 µg/mL of vancomycin to be useful in the detection of VRE with both high- and low-level resistance⁽⁵⁰⁾. Their findings were confirmed by Swensen et al.⁽⁵⁴⁾. The E test has been shown to accurately predict susceptibility to vancomycin (98.7%) and teicoplanin (94.1%) as compared with broth microdilution and disk diffusion⁽⁵⁵⁾, although there are not yet enough published data to allow the test to be recommended as a standard method for antimicrobial susceptibility testing of Enterococcus species. It has thus been recommended that laboratories in areas where VRE is endemic should use strategies such as disk diffusion testing or vancomycin agar screening plates⁽⁵⁰⁾ to augment their automated systems. Agar or broth dilution testing systems to determine MICs are, as indicated, also reliable⁽⁵⁶⁾. Details on the use of alternative agar testing are beyond the scope of this document. Depending upon VRE epidemiology and laboratory testing, it is recommended that laboratories ensure that a mechanism is available to determine vancomycin resistance and high level resistance to penicillin and aminoglycosides of isolates from all clinically important isolates.

[Previous] [Table of Contents] [Next]