Diagnostic in-vitro devices for use in the assaying of biological fluids are provided which include cover plates or backing strips which exhibit hydrophilic properties to assist in transport of the biological fluid or retention of same within the device. Exemplary diagnostic devices include lateral flow devices, microfluidic devices and microtiter plates. The devices may also be comprised of low fluorescent material in order to facilitate any diagnostic determination by use of fluorescent emissions. Hydrophilic properties may be imparted to the cover plates or backing strips by physical or chemical treatment thereof. The cover plates or backing strips may exhibit heat sealable or pressure sensitive properties.