The present invention discloses an assay method for the detection of viable microbial cells in a sample, the assay method comprising the steps of: i) adding an ATP degrading enzyme to a sample suspected of containing viable microbial cells to substantially degrade any extracellular ATP in the sample; ii) adding a phosphate containing compound to the sample to substantially halt action of the ATP degrading enzyme; and iii) subjecting the sample to a detection assay to establish the level of undegraded ATP in the sample to provide an indication of the level of viable microbial cells in the sample.