Abstract: An assay device for the quantitative determination of the concentration of at least one analyte in a liquid sample comprises a planar emitter 2, a planar detector 3, a lateral flow membrane 4 interposed between the emitter 2 and the detector 3, a conjugate pad 5 in fluid communication with a proximal end of the lateral flow membrane 4, the conjugate pad 5 comprising optically detectable tagging particles bound to a first assay component, a sample pad 6 in fluid communication with the conjugate pad 5 and arranged to receive the liquid sample, and a wicking pad 7 in fluid communication with a distal end of the lateral flow membrane 4. The lateral flow membrane 4 is formed from a light transmissive material and is capable of transporting fluid from the conjugate pad 5 to the wicking pad 7 by capillary action.