Surrogate disks for predicting cefotaxime and ceftriaxone susceptibilities of Streptococcus pneumoniae.

Cefotaxime- and ceftriaxone-resistant Streptococcus pneumoniae is now appearing in some medical centers, but 30-micrograms cefotaxime or 30-micrograms ceftriaxone disks are not reliable for detecting such strains. Studies were undertaken to select another cephalosporin disk that might be used as a screening test that could be used in conjunction with a 1-micrograms oxacillin disk. A 30-micrograms cefuroxime disk is proposed: strains with zones > or = 28 mm in diameter are predictably susceptible to cefotaxime and ceftriaxone, and those with smaller zones should be further studied to confirm resistance to either drug. A 30-micrograms ceftizoxime disk may also be used as a screening test with zones > or = 26 mm indicating susceptibility, but cefuroxime disks are preferred.