Emerging multiply resistant enterococci among clinical isolates. II. Validation of the etest to recognize glycopeptide-resistant strains.

Accurate, quantitative susceptibility testing of the enterococci for glycopeptide (vancomycin and teicoplanin) activity has become very important to guide chemotherapy as a result of emerging resistance. Reference dilution tests using broth and agar are generally difficult to perform, or in some commercial forms have demonstrated interpretive error. An alternative method, the Etest has promise as a system with qualitative and quantitative accuracy. Nearly 2000 enterococci from 97 laboratories in the United States were tested using broth microdilution, disk diffusion, and Etest methods. The Etest quantitative accuracy (+/- 1 log2 dilution) compared to the broth microdilution minimum inhibitory concentration was 90.1% (teicoplanin) to 94.1% (vancomycin). The qualitative interpretive accuracy of the Etest ranged from 98.7% for vancomycin to 99.9% for teicoplanin (no false-susceptible errors). All three tests were in remarkable agreement, with < or = 0.8% maximal discord by interpretive category. The Etest appears to be an excellent alternative to reference and standardized methods for producing quantitative activity measurements of glycopeptide susceptibility or resistance.