Comparison of the Vitek GPS-TB card with disk diffusion testing for predicting the susceptibility of enterococci to vancomycin

Objective.-To compare the ability of the Vitek GPS-TB card with disk diffus ion testing for determining the susceptibility of enterococci to vancomycin . Design.-Vitek susceptibility testing was performed using the CPS-TB card an d software version R05.03. Disk diffusion susceptibility testing was perfor med according to National Committee for Clinical laboratory Standards guide lines. When discrepancies occurred between the interpretation of Vitek and disk diffusion, both tests were repeated and the epsilometer test (E test) and agar screen containing 6 mu g/mL vancomycin were performed. Results.-Of 415 isolates tested, 313 were susceptible to vancomycin and 97 were resistant to vancomycin by both test methods. Two isolates were interm ediate by Vitek and resistant by disk diffusion, 2 were intermediate by Vit ek and susceptible by disk diffusion, and 1 was susceptible by Vitek and in termediate by disk diffusion. All but 1 of these falter 5 isolates (interme diate by Vitek and susceptible by disk diffusion) were available for retest ing. On repeat testing, the 2 isolates that were intermediate by Vitek and resistant by disk diffusion were resistant by both methods, the 1 isolate t hat was intermediate by Vitek and susceptible by disk diffusion was suscept ible by both methods, and the isolate that was susceptible by Vitek and int ermediate by disk diffusion was also susceptible by both methods. These res ults were confirmed by E test and agar screen. Conclusion.-We found the results of the GPS-TB card compared well with disk diffusion. However, isolates with intermediate results by Vitek should be retested using another method, such as the E test.