A portable in-vitro (PIV) diagnostic detector operable to perform a fluorescence assay on a sample in one or more detection chambers of a cartridge is provided. The PIV diagnostic detector comprises a first optical module which includes (i) an LED light source for emitting substantially monochromatic light to illuminate a detection zone associated with at least one detection chamber; (ii) an excitation filter interposed between said light source and said detection zone; (iii) a light detector operable to detect fluorescent light emitted by an excited fluorescent label associated with the sample and to measure an intensity of the fluoresced light; and (iv) an emission filter interposed between said light detector and said detection zone. The (PIV) diagnostic detector further comprises a microprocessor operable to process the measured intensity of the fluoresced light to determine whether an analyte is present in the sample; wherein the first optical module is configured such that a longitudinal axis of the light source extends at an oblique angle with respect to a longitudinal axis of the light detector.