RELIABILITY OF THE E-TEST FOR DETECTION OF AMPICILLIN, VANCOMYCIN, AND HIGH-LEVEL AMINOGLYCOSIDE RESISTANCE IN ENTEROCOCCUS SPP

By comparison with apr dilution results, the E test was investigated f or the ability to detect high-level aminoglycoside (gentamicin and str eptomycin), ampicillin, and vancomycin resistance among strains repres enting six enterococcal species. For ampicillin and vancomycin, disk d iffusion results also were obtained. No false high-level aminoglycosid e resistance occurred, and no false gentamicin susceptibility was note d. With the high-range streptomycin E test (2,048 mug), 24% of the 38 resistant strains were falsely susceptible. However, these discordance s could likely be reconciled by adjustments in incubation duration and by using broth microdilution rather than apr screen breakpoint criter ia, or by using the lower-range (1,024-mug) strip. For ampicillin, cat egory results obtained by E test and disk diffusion showed good agreem ent with agar dilation; E test MICs were generally comparable to agar dilution MICs. The E test was more sensitive than disk diffusion for d etecting vancomycin-intermediate strains, but for these strains and th ose exhibiting low-level vancomycin resistance (MIC, 32 to 128 mug/ml) , disk diffusion and E test inhibition zones must be interpreted with caution. Given the reliability of E test for detecting resistance to a nti-enterococcal agents, the decision to use this method should be bas ed on convenience, cost, testing frequency, and satisfaction with curr ently used methods.