An assay stick (7) includes a transparent reaction vessel or tube (14) having one or more microbeads (8) disposed therein. The microbeads (8) have a plurality of unique identification digital codes based on a diffraction grating (12) disposed therein that are detected when illuminated by incident light (24). The incident light (24) may be directed transversely onto the side or onto an end of the tube (14) with a narrow band (single wavelength) or multiple wavelength source, in which case the code is represented by a spatial distribution of light or a wavelength spectrum, respectively. The assay stick (7) may be reused or disposed upon completion of the assay. Alternatively, the beads may be attached to a strip or planar surface. The encoded beads can also provide traceability, quality-control, and authenticity of each bead (8) to its source and/or to the chemistry on each bead (8). Also, the low sample volume of the assay stick allows for faster reactions and better sensitivity.
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