The present invention relates to an in vitro assay method and device that provides for detection and measurement of entities in a fluid sample that can be captured and concentrated in a unitized self-contained enclosed filter apparatus that is analyzed in an optical detection instrument for indications of the entity. It provides for analysis of biological material including cells, their enzymes, or other constituents thereof, that can be identified based on an indicator-generating means. The analyses provided for include detection of the presence of the entity, and changes in the entity over time, such as associated with growth and increasing metabolic activity with an expanding population of cells, or decreasing metabolic activity, for example, due to presence of inhibitory or toxic agents.