A diagnostic test device comprises means for sampling a liquid biological sample. Means is provided for reacting the sample with a reagent to provide one or more visible indicia. A photodetector for scans a detection region to produce a pulsed output indicative of the presence of the one or more indicia. The photodetector is connected to a signal processing circuit for processing the pulsed output. The signal processing circuit produces an output change in response to light intensity variation which is substantially independent of baseline current.