Susceptibility testing of linezolid by two standard methods.

To determine to what extent methodological differences affect susceptibility testing parameters for linezolid, MICs and disk inhibition zones were compared using the methods described by the National Committee for Clinical Laboratory Standards and the British Society for Antimicrobial Chemotherapy for 198 strains of gram-positive cocci. Zones were 4-5 mm larger by the British Society for Antimicrobial Chemotherapy method, but MICs did not differ, except for pneumococci, which were more sensitive when the British Society for Antimicrobial Chemotherapy method was used. This is because incubation in CO2 depresses the activity of linezolid against this species only. Breakpoints for linezolid may need adjusting when testing is by other than National Committee for Clinical Laboratory Standards methods.