Predicting susceptibility of streptococcus pneumoniae to ceftriaxone and cefotaxime by cefuroxime and ceftizoxime disk diffusion testing

In this study, disk diffusion testing with ceftizoxime and cefuroxime was evaluated for use in predicting the susceptibility of Streptococcus pneumoniae to ceftriaxone and cefotaxime. Of the 194 isolates included in this study, 138 were susceptible, 34 were intermediate, and 22 were resistant to cefotaxime by MIC testing; 138 isolates were susceptible, 35 were intermediate, and 21 were resistant to ceftriaxone by MIC testing. A zone of inhibition around the cefuroxime disk of ≥32 mm correctly categorized 101 of 138 isolates as susceptible to cefotaxime and ceftriaxone. A zone of inhibition around the ceftizoxime disk of ≥26 mm correctly categorized 111 of 138 isolates as susceptible to cefotaxime and 114 of 138 as susceptible to ceftriaxone. We conclude that disk diffusion can separate S. pneumoniae isolates susceptible to ceftriaxone and cefotaxime from those that are not susceptible. Isolates not falling into the susceptible category by disk diffusion require additional testing to determine the MIC.