Abstract: The invention relates to a sample testing device for conducting a heterogenous assay, for example an ELISA, in a capillary lumen, using one way flow of sample and wash buffer to move the reaction through the binding, separation and signal measurement steps, thus minimising external intervention. The capillary passage is configured to allow time within different zones for reaction, capture, separation of bound and free fractions, and signal measurement. A combined capture-signal read zone is provided to maximise the capture of signal linked binding member, and signal measurement within the capture zone.