A method for rapidly determining the susceptibility of a microorganism in a urine sample to an antimicrobial agent comprising the steps: a) contacting a first sample containing the microorganism with a first growth medium so as to form a first mixture, wherein the first growth medium is selected to enable the microorganism to proliferate and/or encourage the microorganism cell cycle to commence proliferation; b) contacting a second sample containing the microorganism with a second growth medium so as to form a second mixture, wherein the second growth medium is substantially the same as the first growth medium but further comprises a first antimicrobial agent which may inhibit or slow the proliferation of the microorganism; c) incubating the first and second mixtures, for 30 minutes or less, under conditions suitable to enable or encourage proliferation of the microorganism; d) assessing the first and second mixture, or portion thereof, through a flow cytometer in order to assess one or more biochemical and/or biophysical parameters of the microorganisms in both mixtures; and e) comparing the parameters of the microorganisms in the first mixture with that of the second mixture, after incubation, in order to detect whether the first antimicrobial agent inhibits or slows the proliferation of the microorganism so as to determine the susceptibility of a microorganism to said agent, wherein the parameters are: (i) cell size, cell number and/or cell membrane energisation, and wherein prior to step d), carbocyanine dye, comprising 3,3'-dipropylthiadicarbocyanine iodide (di-S-C3), is added to the mixture or part of the mixture; and (ii) nucleic acid content and/or distribution and said nucleic acid comprises DNA and wherein prior to step d), mithramycin and a nucleic acid stain, comprising SYBR Green or ethidium bromide, are added to the mixture or part of the mixture. a) contacting a first sample containing the microorganism with a first growth medium so as to form a first mixture, wherein the first growth medium is selected to enable the microorganism to proliferate and/or encourage the microorganism cell cycle to commence proliferation; b) contacting a second sample containing the microorganism with a second growth medium so as to form a second mixture, wherein the second growth medium is substantially the same as the first growth medium but further comprises a first antimicrobial agent which may inhibit or slow the proliferation of the microorganism; c) incubating the first and second mixtures, for 30 minutes or less, under conditions suitable to enable or encourage proliferation of the microorganism; d) assessing the first and second mixture, or portion thereof, through a flow cytometer in order to assess one or more biochemical and/or biophysical parameters of the microorganisms in both mixtures; and e) comparing the parameters of the microorganisms in the first mixture with that of the second mixture, after incubation, in order to detect whether the first antimicrobial agent inhibits or slows the proliferation of the microorganism so as to determine the susceptibility of a microorganism to said agent, (i) cell size, cell number and/or cell membrane energisation, and wherein prior to step d), carbocyanine dye, comprising 3,3'-dipropylthiadicarbocyanine iodide (di-S-C3), is added to the mixture or part of the mixture; and (ii) nucleic acid content and/or distribution and said nucleic acid comprises DNA and wherein prior to step d), mithramycin and a nucleic acid stain, comprising SYBR Green or ethidium bromide, are added to the mixture or part of the mixture.