An apparatus, and method of use of such apparatus, for detecting particles of interest that are dispersed in a fluid mix, which typically includes other particles. The apparatus includes an interrogation platform arranged to operate in harmony with an opaque member having an orifice sized to promote single-file travel of the particles there-through. A radiation source is disposed on one side of the opaque member, and a radiation detector is disposed on the other side of the opaque member. Particles of interest are tagged using antibody-binding, fluorescing molecules. Radiation from the source causes the tagged particles to fluoresce in the vicinity of, and passing through, the orifice. The resulting fluorescence is detected by the radiation detector and indicates passage of the particles of interest. One workable opaque member is advantageously included in a thin film assembly carried on a removable and disposable card that is adapted for reception in the interrogation platform.