An assay device is disclosed comprising a housing (10) and a test portion (2), electronic circuitry (30) and an optical assembly (41), each at least partially located in the housing (10). The test portion (2) comprises one or more test zones (2E, 2F) adapted to receive an analyte (22) and a fluorescent label associated with the analyte, the fluorescent label being excitable by excitation light and adapted to emit emission light upon excitation by excitation light. The electronic circuitry (30) comprises one or more light sources (31, 32) and one or more light detectors (33). The optical assembly (41-44) comprises one or more excitation light guides (41, 44) adapted to guide excitation light from the one or more light sources (31, 32) to the one or more test zones (2e, 2f), and/or one or more emission light guides (43, 44) adapted to guide emission light from the one or more test zones (2e, 2f) to the one or more light detectors (33). The sets of emission light guides (43, 44) and excitation light guides (41, 44) may also take the form of individual integral one-piece light guide units where the emission light guide unit rests within the excitation light guide unit.