Criteria for interpretation of agar diffusion susceptibility tests of norfloxacin performed by the ICS method.

Using regression analysis to correlate MIC values and inhibition zone diameters, an attempt was made to establish criteria for interpreting the results of the agar diffusion test with norfloxacin discs (10 mcg). The test was performed on DST agar in accordance with ICS recommendations. The correlation was moderate (r = -0.8547); the regression equation was x = -0.30y + 15.80, where x was log2 MIC + 9 and y the inhibition zone diameter. The position of the MIC breakpoints of norfloxacin and the distribution of the MIC values of the bacterial population did not permit precise inhibition zone breakpoints to be established. To allow a safety margin it is recommended that for the meantime norfloxacin inhibition zones be interpreted as sensitive from 15 mm onwards.