A portable in-vitro, PIV, diagnostic detector operable to perform a fluorescence lateral flow assay on a sample located on a test strip in one or more detection chambers of a cartridge, the PIV diagnostic detector comprising: a first and a second optical module each an LED light source for emitting substantially monochromatic light to illuminate a detection zone associated with at least one detection chamber; an excitation filter interposed between said light source and said detection zone; a light detector, the light detector operable to detect fluorescent light emitted from said detection zone by an excited fluorescent label associated with the sample and to measure an intensity of the fluoresced light; and an emission filter interposed between said light detector and said detection zone; wherein the first optical module and the second optical module are arranged in a row either parallel or orthogonal to the longitudinal direction of the cartridge; and 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 and second optical modules are configured to cause the respective light source to emit light at an oblique angle with respect to the fluorescent light emitted from said detection zone by the excited fluorescent label and detected by the respective light detector; and characterised in that the PIV diagnostic detector is adapted to be removably mountable to the cartridge, wherein, in use, the first and second optical modules are located in a fixed position with respect to the test strip when the test strip is being analysed, by virtue of the PIV diagnostic detector being adapted to be removably mountable to the cartridge, the test strip being provided by the cartridge. a first and a second optical module each an LED light source for emitting substantially monochromatic light to illuminate a detection zone associated with at least one detection chamber; an excitation filter interposed between said light source and said detection zone; a light detector, the light detector operable to detect fluorescent light emitted from said detection zone by an excited fluorescent label associated with the sample and to measure an intensity of the fluoresced light; and an emission filter interposed between said light detector and said detection zone; wherein the first optical module and the second optical module are arranged in a row either parallel or orthogonal to the longitudinal direction of the cartridge; and an LED light source for emitting substantially monochromatic light to illuminate a detection zone associated with at least one detection chamber; an excitation filter interposed between said light source and said detection zone; a light detector, the light detector operable to detect fluorescent light emitted from said detection zone by an excited fluorescent label associated with the sample and to measure an intensity of the fluoresced light; and an emission filter interposed between said light detector and said detection zone; wherein the first optical module and the second optical module are arranged in a row either parallel or orthogonal to the longitudinal direction of the cartridge; and 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 and second optical modules are configured to cause the respective light source to emit light at an oblique angle with respect to the fluorescent light emitted from said detection zone by the excited fluorescent label and detected by the respective light detector; and characterised in that the PIV diagnostic detector is adapted to be removably mountable to the cartridge, wherein, in use, the first and second optical modules are located in a fixed position with respect to the test strip when the test strip is being analysed, by virtue of the PIV diagnostic detector being adapted to be removably mountable to the cartridge, the test strip being provided by the cartridge.