Preliminary disk diffusion susceptibility testing criteria for cefdaloxime (RU29246, HR-916 metabolite), a new orally administered cephalosporin.

Cefdaloxime (formerly RU29246; Hoechst-Roussel Pharmaceuticals Inc., Somerville, N.J.) a new active component of the HR-916 ester, was tested by dilution and two disk (10- and 30-micrograms) diffusion susceptibility tests against 391 clinical isolates. Interpretive criteria were proposed for three potential MIC breakpoints of less than or equal to 1, less than or equal to 2, and less than or equal to 4 micrograms/ml. Analyses by regression line and error rate bounding methods minimized false-susceptible (very major) errors and produced a greater than or equal to 90% absolute interpretive agreement between susceptibility test methods. The less than or equal to 2-micrograms/ml breakpoint seemed optimal when 10-micrograms disks and the available human pharmacokinetics were used. The following inhibition zone diameter criteria were proposed: susceptible, greater than or equal to 19 mm; resistant, less than or equal to 15 mm. These recommendations for clinical trials should remain tentative until additional information about cefdaloxime formulations, pharmacokinetics, and patient outcomes can be correlated with in vitro susceptibility test results.