Abstract: A method of determining the presence or absence in a sample of a biomarker, the method comprising the steps of (a) providing a porous substrate which carries an antigen; (b) contacting the substrate with the sample; (c) contacting the substrate with a composition comprising a secondary antibody; and (d) observing the substrate. The method may enable a very quick, simple test to be carried out to determine whether or not a particular sample contains a biomarker, for example an antibody indicative of infection with a mycobacterial disease. A kit and a device for determining the presence or absence in a sample of a biomarker are also described.