Evaluation of the Vitek 2 system for susceptibility testing of Streptococcus pneumoniae isolates

Vitek 2 (bioMerieux, France) is a new commercial system that allows rapid i dentification and rapid determination of the minimum inhibitory concentrati on (MIC) of Streptococcus pneumoniae by monitoring the growth kinetics of t he organisms in microwells. The accuracy of the Vitek 2 system ill suscepti bility testing was evaluated by determining the MICs of 50 penicillin-susce ptible and 150 intermediate or penicillin-resistant Streptococcus pneumonia e isolates and comparing the results with those obtained using the agar dil ution method. The essential agreement between the Vitek 2 system and the re ference method was 91% for penicillin, 93% for cefotaxime and ceftriaxone, and more than 94% for amoxicillin, erythromycin, ofloxacin, co-trimoxazole, tetracycline, and imipenem. One very major error (1.1%) and one major erro r (0.9%) were obtained for tetracycline. The minor error rate for penicilli n of 19.3% was mainly due to intermediate category isolates (n = 29) being identified as resistant and susceptible isolates (n = 6) being identified a s intermediate by the commercial system. The minor error rates for amoxicil lin, cefotaxine, ceftriaxone, imipenem, and ofloxacin were 25.4%, 25.4%, 29 .4%, 19,2%, and 31.5%, respectively. Vancomycin, tetracycline, co-trimoxazo le, and erythromycin showed minor error rates of 0-6.1%. In conclusion, Vit ek 2 shows good agreement with the reference method, as demonstrated by the low numbers of major errors, but it has a tendency to overestimate MICs, r esulting in minor errors.