line 24 to calculate the amount of analyte 40 present in the sample or solution this comparison step may occur visually or with the aid of a reading device not shown wicking pad 28 receives the fluid that has migrated through membrane 23 the membrane 23 or solid support which is employed in the assay may be a cellulose ester nitrocellulose is known provides good results in some applications it should be understood that the term u201c nitrocellulose u201d refers to nitric acid esters of cellulose which may be nitrocellulose alone or a mixed ester of nitric acid and other acids in particular aliphatic carboxylic acids having from one to seven carbon atoms although nitrocellulose may be a suitable material for producing the membrane it is to be understood that other materials may also be employed for such solid supports including but not limited to nylon rayon and the like in accordance with a particular preferred embodiment the pore size of the solid support is such that the probe when bound to the analyte remains on the surface of the membrane 23 thus for example good results have been obtained with nitrocellulose having a pore size of from about 0 1 to 0 5 microns it is to be understood that the invention can be configured for detecting a broad range of analytes including therapeutic drugs drugs of abuse hormones vitamins proteins including antibodies of all classes.