Abstract: A method for detecting a target substance which includes collecting a substance sample; introducing the substance sample into a substance card having at least one reagent responsive to the presence of the target substance and having a light-transmissive chamber; inserting the substance card into a substance detector device having a photosensor and adapted to receive the substance card; mixing the substance sample with the reagents for a preselected mixing period, thus producing a measurand having a target substance reaction; streaming the measurand through the light-transmissive chamber and illuminating the measurand with a quantity of light for a selectable analysis period, an optical characteristic of the measurand can be measured with the photosensor; and selecting the target substance reaction as a positive reaction or a negative reaction, responsive to the optical characteristic. If the target substance reaction is a negative reaction, the device operator is signalled thereof.