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 e valuated for use in predicting the susceptibility of Streptococcus pneumoni ae to ceftriaxone and cefotaxime. Of the 194 isolates included in this stud y, 138 were susceptible, 34 were intermediate, and 22 were resistant to cef otaxime 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 greater than or equal to 32 mm correctly cat egorized 101 of 138 isolates as susceptible to cefotaxime and ceftriaxone. A zone of inhibition around the ceftizoxime disk of greater than or equal t o 26 mm correctly categorized 111 of 138 isolates as susceptible to cefotax ime and 114 of 138 as susceptible to ceftriaxone. We conclude that disk dif fusion can separate S. pneumoniae isolates susceptible to ceftriaxone and c efotaxime from those that are not susceptible. Isolates not falling into th e susceptible category by disk diffusion require additional testing to dete rmine the MIG.