Interpretation criteria for sensitivity testing of ceftriaxone in agar diffusion test according to the DIN 58940 method.

On the basis of a regression analysis of the correlation between minimal inhibitory concentration (MIC) and inhibitory zone diameter, interpretative criteria are established for the agar diffusion test with ceftriaxone disks (30 micrograms) in DST agar according to the DIN method. The correlation is good (r = -0.9242); the regression equation is x = -0.43 y + 20.53 (y = inhibitory zone diameter, x = log2MIC + 9). Following appropriate discussion, provisional MIC breakpoints of 8 and 32 mg/l are proposed for ceftriaxone. On the basis of these threshold values, the following criteria are established for interpreting the inhibition zone of ceftriaxone: resistance (corresponding to an MIC of 64 mg/l and more up to 15 mm; moderate sensitivity (corresponding to an MIC of 16-32 mg/l) 16-19 mm; sensitivity (corresponding to an MIC of up to 8 mg/l) 20 mm and more.