Selection of zone size interpretive criteria for disk diffusion susceptibility tests of three antibiotics against Streptococcus pneumoniae, using the New Guidelines of the National Committee for Clinical Laboratory Standards.

Disk diffusion and broth microdilution susceptibility tests were performed with cefotaxime, ceftriaxone, telithromycin, and erythromycin (control) against 407 selected isolates of Streptococcus pneumoniae. Scattergrams were prepared from the results of these tests, and the current NCCLS guidelines for setting disk diffusion test interpretive criteria were applied. Erythromycin zone diameter breakpoints were confirmed. Telithromycin interpretive criteria for the disk test could be easily set with acceptable discrepancy rates. For cefotaxime and ceftriaxone, the minor discrepancy rates for MICs in the intermediate category +/- 1 dilution were far in excess of the acceptable 40% limit, i.e., 52 and 71%, respectively. We conclude that the 30-microg disk of these two drugs cannot be reliably used to test pneumococci.