A microfluidic assay device that defines a micro-fluidic flow channel (44) having a flow axis, in which a series of discrete, axially-spaced apart, transparent hollow flow elements (32) are secured in fixed position, each flow element having at least one axially-extending flow passage through its interior, assay capture agent fixed to the interior surface of the elements for capture of an analyte in liquid flowing through the interior of the flow elements, the device constructed to enable light to be transmitted out of the elements for reading of fluorescence from captured analyte, wherein: the exterior axially-extending surfaces of the flow elements are free of active capture agent, while at least part of the interior surfaces carry deposits of active capture agent exposed to flow through the elements.