Detection of analytes

The invention relates to an improved method of detecting analytes in a sample of body fluid. In this method, the sample is contacted with a specific binding agent capable of forming a specific binding complex with the analyte wherein the specific binding agent is immobilised on a porous solid substrate. The presence of specific binding complex is then detected. The method of the invention has the additional step of wiping the surface of the substrate before detection takes place in order to remove unadsorbed contaminants. The unadsorbed contaminants may be visualised on the surface of the substrate using a colouring agent so that it is easy to tell when the contaminants have been removed.