A method and apparatus for determining the minimum inhibitory concentration of an antibiotic for a target microorganism is provided. The method includes the steps of: (a) providing a microorganism growth medium; (b) providing a sensible reagent, which includes an antibiotic mixed with a marker, the marker having a signal with a magnitude proportional to the marker's concentration; (c) incorporating the reagent into the growth medium, in a manner that creates a gradient of concentrations of the antibiotic and marker within the growth medium; (c) inoculating the growth medium with the target microorganism; (d) incubating the inoculated growth medium for a period of time sufficient for the target microorganism to grow a detectable amount on a first section of growth medium; (e) determining a growth boundary between the first section of growth medium having detectable target microorganism growth and a second section having substantially no detectable target microorganism growth; (f) measuring the signal magnitude at the growth boundary; and (g) determining a minimum inhibitory concentration of the antibiotic using the measured signal magnitude.