Comparison of cefadroxil and cephalothin disk susceptibility test results.

Diffusion susceptibility tests with 30-micrograms cefadroxil disks and 30-micrograms cephalothin disks were evaluated. For both agents, the same zone size interpretive criteria were recommended (less than or equal to 14 mm for resistance and greater than or equal to 18 mm for susceptibility). Tests were performed with 904 bacterial isolates, and the data were examined to determine whether the two cephalosporins might be used interchangeably for purposes of in vitro susceptibility testing. When Haemophilus influenzae, Listeria monocytogenes, and methicillin-resistant staphylococci were evaluated, the two agents differed significantly. For testing other species, a cephalothin disk or cephalothin MIC could be used for predicting susceptibility or resistance to cefadroxil.