Abstract: A method for detecting biological activities in a specimen, for instance a blood sample, employing a sealable container with a culture medium, into which the sample is introduced, wherein metabolic processes are enhanced in the presence of microorganisms in the sample, thereby causing changes to take place in the concentrations of the substances subject to such processes. The concentration changes are measured by optodes that are in direct contact with the sample, and by an excitation and detection assembly assigned to these optodes, to which is connected an evaluation unit for determining concentration changes of the substances over time.