The invention concerns a portable device for reading a fluorescent-labelled, membrane based assay, to detect the presence of an analyte in a sample. The portable device comprises a light sealed housing which includes a receiving member to receive a membrane based assay. The light sealed housing comprises a first light source for illuminating a first portion of the membrane with light, a first photodetector, a light guide and a microprocessor. The first photodetector is operable to detect fluoresced light emitted by an excited fluorescent label captured by a reagent laid down in the first portion of the membrane and to measure an intensity of the fluoresced light. The light guide guides illuminated light from the first light source to the first portion of the membrane and guides fluoresced light emitted by the excited fluorescent label to the first photodetector. The microprocessor is operable to process the measured intensity of the fluoresced light to determine whether the analyte is present.