In procurements of detection instruments for biological warfare agents to the armed forces or civilian authorities dealing with public safety, it is an increasing demand of finding objective criteria to compare instruments from different vendors. It is also important for manufacturers that evaluations at different laboratories can give similar result. This is the ground for a trilateral collaboration between the three national CBRN laboratories at TNO (The Netherlands), DRDC Suffield (Canada) and FOI (Sweden). The idea has been to work together and learn from each other. The first co-operation was on a test to measure the limit of detection (LOD) using the TNO aerosol chamber. The limit of detection presented in this work is a measure of the physical limitations of the instrument and shows the best achievable performance of the detector. The report show that the selected method of calculating the LOD is straight forward and should be relatively easy to apply to all tests and evaluations of biological detection devices.