E test versus agar dilution for antimicrobial susceptibility testing of viridans group streptococci

Viridans group streptococci (VGS) are commonly isolated from the blood of h ospitalized patients. The E test represents a convenient method for determi ning the MICs for VGS, but for this purpose it has not been well validated against reference methods. In this study, 180 unselected VGS isolates were identified to a species level, and the MICs of penicillin, cefuroxime, cefo taxime, and vancomycin were determined by both agar dilution and the E test . Available data regarding demographic and laboratory variables for each VG S bacteremic episode were collected, the significance of each VGS isolate w as assessed, and the associations between and among laboratory and clinical variables were investigated. Among all VGS isolates, 68.3% (median of thre e runs) were found to be fully susceptible to penicillin by agar dilution. The E test and agar dilution showed average agreements (within +/-1 dilutio n) of 92.2% for penicillin, 95.7% for cefuroxime 91.3% for cefotaxime, and 86.7% for vancomycin, Agreements over serial E tests and serial agar diluti ons were excellent for beta-lactam agents (intraclass correlation coefficie nts, >0.9) but less impressive for vancomycin, Very major error rates for t he E test were less than or equal to 0.7%, and combined major and minor err or rates were within acceptable limits for all antimicrobial agents tested. Lysis-centrifugation culture methods were more often associated with clini cally insignificant VGS isolates; otherwise, no associations between clinic al and laboratory variables were noted.