Determination of interpretive breakpoints for ceftazidime disc-diffusion susceptibility testing using single-strain regression analysis.

Interpretive breakpoints for ceftazidime disc-diffusion susceptibility testing were determined using single-strain regression analysis (SRA). Regression lines were determined for a total of 58 strains representing 15 species, from inhibition zone diameters obtained for discs containing six different ceftazidime concentrations. Statistical analysis for excluding non-linearity of test-results was performed. A minimum of five tests on consecutive days was required for maximal precision of regression analysis according to the SRA-method. Calculated regression lines showed similarities within individual and groups of bacterial species. A minimum of five strains could be used to represent these groups. Interpretive breakpoints according to recommended MIC-limits were determined for each species taking into consideration confidence limits for zone correlates of MIC-values. Single-strain regression analysis for the determination of interpretive breakpoints for ceftazidime disc-diffusion susceptibility testing in individual laboratories.