A method for extracting ATP from a biological sample is disclosed. The method involves introducing a cationic extractant and an anionic substance and then extracting ATP. The method may be used to assay for the presence of ATP in a biological sample or to determine the amount of ATP extracted from a biological sample. The method is particularly useful in detecting contamination on surfaces and in food products. A reagent, a test device and a test kit that involve the use of the method to detect contamination are also disclosed.