New formula for calculating antibiotic critical concentration by the disk diffusion method.

New formula was designed for calculation of the antibiotic critical concentration: Cc = (1/pi Bh)e-A/B where h is agar depth, B is the slope, A is the intercept. The formula is derived from the second Fick's law. Neither diffusion constant nor critical time data apply. Three or more disks containing varied concentrations of the same antibiotic are used. Regression analysis of square of zone of inhibition radii is performed and subsequently the critical concentration is computed.